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Quercetin
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In one pass If you ate an onion or an apple today, or drank a cup of tea, you have already taken in a little quercetin.
Educational content, not medical advice — consult a clinician.
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Chapter 1
A common flavonoid in food
If you ate an onion or an apple today, or drank a cup of tea, you have already taken in a little quercetin. It is the most common and most studied of the flavonoids — a large family of colored polyphenol molecules in plants — and if you only get to know one flavonoid, it is usually this one.
In food it mostly carries a sugar attached (the glycoside form). After you swallow it, the gut first cuts the sugar off, and only then is quercetin absorbed; right after that, the liver quickly attaches other chemical groups to it. So what flows in your blood is no longer the same thing you put in your mouth — every later question about whether it works comes back to this.
The other thing that matters is scale. A typical diet supplies only a little quercetin a day, while one supplement dose is 500-1000 mg, many times more than food provides. That is no longer topping up a nutrient; it is closer to taking a weak drug. Whether the little bit in food and the big handful in a capsule can be treated as the same thing is the central question of this story.
In food it mostly carries a sugar attached (the glycoside form). After you swallow it, the gut first cuts the sugar off, and only then is quercetin absorbed; right after that, the liver quickly attaches other chemical groups to it. So what flows in your blood is no longer the same thing you put in your mouth — every later question about whether it works comes back to this.
The other thing that matters is scale. A typical diet supplies only a little quercetin a day, while one supplement dose is 500-1000 mg, many times more than food provides. That is no longer topping up a nutrient; it is closer to taking a weak drug. Whether the little bit in food and the big handful in a capsule can be treated as the same thing is the central question of this story.
Evidence · What flavonoid-diet studies can show
"Eat more flavonoid-rich foods" is common health advice, but it is important to separate the effect of a whole dietary pattern from the effect of quercetin as a single molecule.Population studies (dietary flavonols vs chronic disease):
In observational studies, people who eat more foods rich in flavonols (the group quercetin belongs to) have a lower risk of some chronic diseases. This is an observed association; it does not show that flavonols caused it.People who eat more of them also eat more fruit, vegetables and tea and have healthier diets overall. This is an association with the whole dietary pattern, not an effect of quercetin on its own.No randomized trial has shown that supplementing quercetin alone reproduces the association seen with dietary flavonols.
The core contradiction:
Food: 100 mg of flavonols embedded in fruit and vegetables, together with more than 1000 other plant compounds, plus fiber, vitamins and minerals.Supplement: 500 mg of pure quercetin, isolated from everything else.Population studies can only ever observe the former.
In practice:
Getting it from food is easy and cheap: a cup of green or black tea, a serving of onion (in a salad or soup), an apple with its skin or a handful of blueberries — that is real dietary flavonol.You do not need a quercetin supplement to get there.Capers have the highest quercetin content, but that is not an instruction: unless you already like them, there is no need to buy them for this.
One more point: the onions, tomato sauce and olive oil of a Mediterranean diet supply many kinds of flavonoids and other plant compounds; a supplement supplies one. That variety is exactly what a supplement cannot copy.
Chapter 2
Can it work as an antihistamine?
The claim that quercetin is a natural antihistamine is half true as a mechanism and weak in the clinic. In the lab it can steady the membrane of mast cells so that less histamine leaks out; in people, hay-fever trials are mostly small and have shown only mild improvement, with no large, high-quality trial behind them.
Second-generation antihistamines start working within half an hour and last all day; quercetin takes several days to work and cannot replace first-line drugs for moderate-to-severe allergy. At high doses it also runs into the thyroid, drug interactions and iron absorption — which matters more to remember than its effect.
An allergic asthma attack or a severe allergic reaction (tightness in the throat, difficulty breathing) needs medical care immediately; that is not something any supplement can handle.
Second-generation antihistamines start working within half an hour and last all day; quercetin takes several days to work and cannot replace first-line drugs for moderate-to-severe allergy. At high doses it also runs into the thyroid, drug interactions and iron absorption — which matters more to remember than its effect.
An allergic asthma attack or a severe allergic reaction (tightness in the throat, difficulty breathing) needs medical care immediately; that is not something any supplement can handle.
Evidence · Does the natural antihistamine claim hold?
The supplement market's favorite line is that quercetin is a natural antihistamine. As a mechanism that is half true; in the clinic it is weak.First, the half that is true. During an allergic flare, immune cells called mast cells burst like a punctured water bag and release histamine all at once, causing sneezing, a runny nose and itching. In the lab, quercetin can steady that cell membrane so that less histamine leaks out, and it also turns down several pro-inflammatory signals. This has been repeated many times in cells and mice, and at that level it is fairly solid.
The problem is in people. Hay-fever trials are mostly small (usually under 50 people) and have shown only mild improvement; there is still no large, high-quality trial behind it. A recent review put it politely: early results are promising and larger studies are needed — in plain terms, the evidence is not there yet.
Put it beside a real first-line drug and the picture is clear: second-generation antihistamines (cetirizine, loratadine and the like) are backed by hundreds of trials, start working in half an hour, last all day and cost pennies a day; quercetin has weak evidence, takes several days to work, and costs far more per day.
So in practice it is simple:
Only mild seasonal allergy, and you prefer a natural option: quercetin might help a little and will not hurt you, but do not expect quick relief.Moderate-to-severe allergy: go straight to a second-generation antihistamine or a steroid nasal spray; do not use quercetin as a substitute.Allergic asthma or a severe allergic reaction: this is not a quercetin situation at all — seek medical care immediately.
If you want options that are just as natural, a HEPA air filter against pollen and dust mites and saline nasal rinses are often overlooked; both directly reduce your contact with allergens or wash them out.
Safety · The real risks of high-dose quercetin
The "quercetin is a natural antihistamine" marketing hides a gap in its logic:Where the gap is:
A similar mechanism does not mean clinically equivalent. Vitamin E and chromium both fell at this step; it is a trap that recurs in nutrition science.A weaker clinical effect does not mean gentler and more natural; it just means weaker.Gentle also means slow to start, large doses, wide variation between people and poor predictability.
Real risks that are often overlooked (most of what follows comes from cell and animal studies, and how much clinical weight it carries in people is unclear; but if you take these drugs, ask your doctor first):
Thyroid: in experiments, high-dose quercetin inhibits thyroid peroxidase (, an enzyme needed to make thyroid hormone) and affects hormone synthesis. People with hypothyroidism or subclinical hypothyroidism should use caution.Drug interactions: it inhibits a drug-processing enzyme in the liver (, part of the cytochrome P450 family).It may raise blood levels of statins, calcium-channel blockers and immunosuppressants such as cyclosporine.Together with warfarin, it may increase the risk of bleeding.It may interfere with the metabolism of chemotherapy drugs (cyclophosphamide, doxorubicin).Iron absorption: quercetin can bind iron ions and may reduce absorption of the kind of iron found in plant foods; people with iron deficiency and menstruating women should keep this in mind.Pregnancy: data are insufficient; generally not recommended.Before surgery: because it may increase bleeding, the usual conservative practice is to stop 2 weeks before an operation (the same as for fish oil).
Acute toxicity: quercetin itself has low acute toxicity; at high doses the common side effects are stomach upset, headache and tingling in the hands and feet.
So: "natural means safe" is an illusion. Quercetin is not lethal, but it is certainly not free of side effects, and people who take common medications and plan to take high-dose quercetin should be especially careful.
A more reasonable stance:
Get it from food: a lifetime of onions, tea and apples is safe.Supplement doses (500 mg a day or more): treat it as a weak drug, not a gentle wellness product — that is the more accurate frame.
Chapter 3
Poor absorption, small sports effect
Quercetin's biggest flaw is how poorly it is absorbed. Only a small fraction actually reaches the blood, and the liver immediately remodels it, so blood levels sit far below the concentrations used in those impressive dish experiments. Its mechanisms therefore lose much of their force by the time they reach people; formulation tricks can lift absorption somewhat, but they do not change the bigger picture that its clinical effects are weak.
Exercise is the area where it has been studied most. A pooling 11 trials and 254 people found that, combining endurance and maximal oxygen uptake, quercetin beat placebo by only about 2% on average — statistically significant, but somewhere between trivial and small. Next to creatine and caffeine, it does not make the short list of sports supplements actually worth taking.
Exercise is the area where it has been studied most. A pooling 11 trials and 254 people found that, combining endurance and maximal oxygen uptake, quercetin beat placebo by only about 2% on average — statistically significant, but somewhere between trivial and small. Next to creatine and caffeine, it does not make the short list of sports supplements actually worth taking.
Evidence · Poor absorption, a tiny sports effect
There is no discussing quercetin without its main flaw: it is very poorly absorbed. Every argument over whether it works at all has its root here.After you swallow it, the small intestine strips off its sugar and absorbs it, but only a small fraction actually reaches the blood — typical of polyphenols. As soon as it is absorbed, the liver wraps it in another layer and remodels it, so blood levels often stay below 1 µmol/L. The impressive antioxidant and anti-inflammatory results from the lab used concentrations tens to hundreds of times higher. In other words, what holds in a Petri dish is simply under-dosed in your body.
The market has a few tricks to rescue it: an enzyme-treated form (EMIQ, isoquercitrin) is absorbed 5-10 times better, and liposomes and nano-carriers help a little, but none of them changes the bigger picture that its clinical effects are weak.
Exercise is where it has been studied most. What first made it popular was Davis 2010: 12 healthy people who did not exercise regularly took 500 mg twice a day for 7 days in a randomized, double-blind, crossover design; maximal oxygen uptake was 3.9% higher than on placebo, and time to exhaustion on a bike was about 13% longer. But the sample was far too small, and it needs to be replicated.
Later, Kressler 2011 pooled 11 trials and 254 people in a (median dose 1000 mg a day, median duration 11 days) and pinned it down: combining maximal oxygen uptake and endurance performance, quercetin beat placebo by only about 2% on average — statistically significant, but somewhere between trivial and small. The analysis also compared the endurance performance and maximal oxygen uptake subgroups specifically and found no difference (P = 0.69): it is not that one works and the other does not; both show only this much.
Set against the supplement rankings it is even clearer: for strength and power, creatine gives a clearly larger boost; for endurance, caffeine's average effect is also larger; quercetin does not make the top five. If you really want better performance, caffeine, creatine, enough carbohydrate and protein, and good sleep all give far more for the money.
In practice · If you do try quercetin
If you decide to try quercetin (for seasonal allergy or for exercise), this is the more reasonable way to use it.1. Choose the form:
First choice, isoquercitrin (EMIQ) or a phospholipid complex (phytosome): absorbed 5-10 times better than the plain form.Second choice, a combination with vitamin C or bromelain: absorption is slightly better; any "synergistic anti-inflammatory" effect is only a hypothesis.Avoid: cheap single-ingredient quercetin dihydrate, which is poorly absorbed.
2. Dose:
What trials used: 500-1000 mg of plain quercetin a day (the median dose in the above was 1000 mg).Switching to a better-absorbed form: if absorption is 5-10 times higher, 50-100 mg of EMIQ roughly matches the amount absorbed from 500 mg of plain quercetin. That is inferred from absorption; no trial has compared the effects directly.Allergy season: the usual advice is to start 2 weeks ahead and continue until the season is over.Exercise: a common approach is 1000 mg a day for 2 weeks, then 1 week off before repeating; no trial has compared cycles.Do not take more than 1000 mg a day long-term.
3. When to take it:
With a meal: it dissolves poorly in water and is absorbed better alongside a meal that contains fat.
4. Do not combine with:
Warfarin, apixaban, rivaroxaban (bleeding risk).Cyclosporine, tacrolimus, sirolimus (all processed by ).Statins (CYP3A4).Thyroid medication: quercetin inhibits and may interfere with thyroid hormone synthesis.During chemotherapy or radiotherapy.
5. Expectations:
Slow to start: effects usually take 1-4 weeks to appear.Mild: "slightly fewer symptoms", not "symptoms gone".Wide variation between people: the enzymes that process it (CYP, UGT) vary genetically, and gut bacteria and absorption also differ from person to person.
6. In cycles, or long-term:
Seasonal (spring or autumn pollen, 4-8 weeks): temporary use is reasonable.Heavy training blocks (a training cycle of 2-8 weeks): temporary use is reasonable.Long-term wellness or anti-aging: current data do not support it. The experimental combination of dasatinib plus quercetin (D+Q) is a different matter, covered in the chapter New research on clearing aged cells.
Overall, quercetin is a supplement that may help a little, with a weak effect: not "a scam", and not "essential". Certainty of evidence: low (mostly small randomized trials, small effects).
Chapter 4
New research on clearing aged cells
Quercetin's most interesting new direction is an experimental combination with dasatinib, a prescription drug for leukemia, designed to clear senescent cells. Senescent cells stop dividing but refuse to die, and they keep leaking inflammatory factors that drag down the tissue around them.
In mice, the results were striking. In people, there is so far only one small study of 9 patients without a control group, which saw markers of senescent cells fall — a measurement at the level of cells, not whether any disease got better.
So do not try this combination yourself: dasatinib is a prescription drug for leukemia, with plenty of side effects. The "anti-aging blends" sold in shops lack exactly that ingredient, and they are not the trial regimen.
In mice, the results were striking. In people, there is so far only one small study of 9 patients without a control group, which saw markers of senescent cells fall — a measurement at the level of cells, not whether any disease got better.
So do not try this combination yourself: dasatinib is a prescription drug for leukemia, with plenty of side effects. The "anti-aging blends" sold in shops lack exactly that ingredient, and they are not the trial regimen.
Evidence · Striking in mice, a first proof in people
Quercetin's most interesting new direction is as a cleaner: technically a senolytic, a drug that specifically kills senescent cells. It is one of the few experimental therapies in the biology of aging that has already reached human trials.First, what a senescent cell is. A cell that has been damaged but not quite killed can get stuck in a zombie state: it no longer divides, yet it refuses to die. The trouble is that it does not sit quietly: it keeps pumping out inflammatory factors (this secretion package is called the senescence-associated secretory phenotype, SASP), dragging down the healthy cells around it and feeding low-grade inflammation throughout the body. With age these cells pile up in skin, lung, brain, bone and joints, and they have been linked to osteoarthritis, pulmonary fibrosis and diabetic kidney disease.
Hence the senolytic idea. A 2015 study from Kirkland's team at the Mayo Clinic found that senescent cells survive on a few pro-survival pathways; cut them and senescent cells die while normal cells are spared. Dasatinib (D) worked against senescent human fat-cell precursors, quercetin (Q) was more effective against senescent human blood-vessel lining cells, and the two together lowered the burden of senescent cells in old, irradiated and prematurely aging mice (Zhu 2015, cells and mice only). Later, in Xu 2018, giving old mice D+Q intermittently extended their survival after the start of treatment by 36%.
In people, an earlier first clinical trial, in patients with idiopathic pulmonary fibrosis, saw physical function improve; the first study to show directly that senescent cells fall in the human body was Hickson 2019: an open-label phase 1 pilot study with no control group, in which 9 patients with diabetic kidney disease took dasatinib 100 mg plus quercetin 1000 mg a day for 3 days, with biopsies 11 days after the last dose. Senescent-cell markers in fat and skin fell, and so did several inflammatory factors in the blood. The concept was confirmed in people for the first time — but the sample was tiny and uncontrolled, and what was measured was a cellular marker, not whether any disease got better. Trials in Alzheimer's disease, pulmonary fibrosis and osteoporosis have since begun, and none yet supports using it to prevent or treat anything.
For you personally, the answer right now is clear: do not try D+Q yourself. Dasatinib is a prescription drug for leukemia, with plenty of side effects. Some think that over the next five to ten years this may be one of the most promising clinical directions in aging research; another one worth watching is fisetin, a flavonoid that among common foods is highest in strawberries, which some think may be a safer Q — trials are running.
What makes it worth mentioning is that, for the first time, quercetin has been lifted from an ordinary wellness supplement into the ranks of experimental anti-aging drugs — one of the rare directions in the supplement field where a mechanism has actually made some headway toward the clinic. The story itself has become more precise too: from "eat more apples to fight aging" to "clear senescent cells with a specific combination at a specific time". But if you want to age well, the most powerful tools right now are still exercise, plus eating well, sleeping well and keeping tobacco and alcohol in check.
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Myth · Is a senolytic blend the trial regimen?
The idea of clearing senescent cells is getting hotter, and the marketing is catching up. Here is how to keep a clear head.What is on sale now (2026):
Online shops now carry all kinds of "anti-aging blends": quercetin in various combinations with fish oil, curcumin or fisetin.A typical blend contains quercetin 250-500 mg, plus something like piperine, berberine or an precursor.The real D+Q clinical regimen is prescription dasatinib plus quercetin at the dose set by the trial — not an over-the-counter product.
Point by point:
Contains quercetin, fisetin, NAD precursors: chemically, yes, these are ingredients used in aging research.Whether the effect equals D+Q in the trials: almost certainly not, because the dasatinib is missing.Whether it is harmful: mostly safe; but there are no long-term data on taking quercetin 500 mg plus fisetin every day for years.Whether it is worth 60-100 US dollars a month: given the current strength of evidence, not really.
A more reasonable approach:
Follow: clinical trial updates on senolytics (clinicaltrials.gov); the Mayo Clinic and the Buck Institute are the main research centers.Food: eat more onions, apples, strawberries and blueberries, and drink tea. The flavonoid doses in food are far below trial doses and cannot be treated as an anti-aging drug, but the benefits of these foods are real in their own right, and they are safe.Exercise: in studies of old mice and some older people, exercise also lowered markers of senescent cells, and its benefits go far beyond that one — more solid than any supplement.Hold off for now: on commercial senolytic blends, until large randomized trials report.
What may change within 5 years:
D+Q (or a successor), if a phase III randomized trial succeeds, would become a prescription drug for rheumatoid disease, idiopathic pulmonary fibrosis or common chronic diseases of aging — not an over-the-counter supplement.Fisetin at 100 mg a day long-term, if proven safe and effective, might become the first recognized over-the-counter senolytic.Combinations of NAD precursors (, ) with senolytics are at an early exploratory stage.
Clearing senescent cells is one of the few genuinely promising directions near the supplement world, but it is not yet mature enough for individuals to use with confidence. Keeping track without following blindly is the most rational stance right now.
Chapter 5
The COVID claims and how to use it
During the pandemic years, quercetin rode a wave of mechanism speculation turned commercial hype. In test tubes and computer simulations it could block a key enzyme of the coronavirus, and there was a hypothesis that it helps zinc get into cells; both sounded plausible. But no independent large trial was ever completed, and guidelines did not recommend it for preventing or treating COVID-19.
What actually bent the curve were vaccines and antiviral drugs, not supplements. The story is worth remembering because it replays nutrition's most classic cycle: an enticing mechanism, media amplification, rising sales — and clinical evidence that never caught up.
What actually bent the curve were vaccines and antiviral drugs, not supplements. The story is worth remembering because it replays nutrition's most classic cycle: an enticing mechanism, media amplification, rising sales — and clinical evidence that never caught up.
Myth · The COVID hype, and use by situation
During the pandemic years, quercetin rode a wave of COVID-19 marketing — the most recent example of a nutritional mechanism hypothesis being overhyped commercially.It began with a few laboratory studies: in test tubes and computer simulations, quercetin could partly block a key enzyme of the coronavirus (the main protease) and interfere with the virus binding to a cell receptor; there was also a "zinc carrier" hypothesis, which said it helps zinc get into cells, where zinc then suppresses viral replication. The mechanisms all sounded plausible. Then social media amplified it: some doctors and influencers promoted "quercetin plus zinc" as a natural version of hydroxychloroquine.
The real clinical evidence never caught up. A few small trials (50 to 150 people) suggested it might slightly shorten symptoms, but at the height of the pandemic not a single independent, multicenter, placebo-controlled large trial was completed. The World Health Organization, the US National Institutes of Health and the US Centers for Disease Control and Prevention did not recommend it for preventing or treating COVID-19. What actually bent the curve were vaccines and antiviral drugs such as nirmatrelvir/ritonavir (Paxlovid), not supplements.
When panic over an acute disease meets an unproven natural remedy, people are especially prone to biased judgment.
Pulling quercetin together as a whole, here is how to use it by situation:
Seasonal allergy: you can try plain quercetin at 1000 mg a day for two weeks; if you feel it helps, continue, and if not, stop.During endurance training: it can be an extra, 1000 mg a day in two-week cycles, but the effect is small and it is not a main lever.To age well: stay away from commercial anti-aging blends for now, and get exercise, diet and sleep right first.To prevent or treat COVID-19: follow vaccines and antiviral drugs; do not rely on quercetin.Everyday wellness and inflammation: no supplement needed — some onions, apples and tea are enough.
Overall: the certainty of evidence is low (small trials, small effects), its safety is moderate (there are real drug interactions), and its value for money is low (food and exercise are better and cheaper); the senescent-cell line is worth watching, but it is still early. Should you buy it now? For most people, no — unless you have a clear use for it, and then only as a short trial.
Background · Which flavonoids are worth knowing
Quercetin is the flagship flavonoid, but the whole flavonoid family is worth a quick introduction:The main subgroups, representative molecules and foods:
Flavonols: quercetin, kaempferol, myricetin.Foods: onions, apples, tea, berries, grapes.Flavones: apigenin, luteolin.Foods: parsley, celery, chamomile, thyme.Flavanones: hesperidin, naringenin.Foods: the layer between the peel and the flesh of citrus fruit.Flavanols: catechins (including ) and proanthocyanidins.Foods: green tea (the richest in EGCG), dark chocolate, apples, berries.Isoflavones: genistein, daidzein.Foods: soy (tofu, soy milk, natto, edamame); these are phytoestrogens.Anthocyanins: variants of many colors.Foods: blueberries, blackberries, black rice, purple sweet potato, red and purple fruit.
The simplest checklist for covering the whole flavonoid spectrum from food:
Green or black tea: flavanols and flavonols.Onions, apples: flavonols.Citrus (orange, lemon, grapefruit, with the white pith): flavanones.Soy foods (tofu, soy milk): isoflavones.Berries (blueberries, blackberries, strawberries): anthocyanins and flavonols.Dark chocolate with 70% or more cocoa: flavanols.Red wine also contains resveratrol and flavanols, but drinking for the flavonoids is a bad trade: the risks of the alcohol itself cancel that small benefit.
Eating 3-5 of these a day (at least 3) covers flavonoids well, more broadly than any single-supplement plan.
A special note on soy isoflavones:
They are phytoestrogens and bind weakly to estrogen receptors (ER).Possible benefits: mild relief of menopausal hot flashes, a small protective effect on and a small cardiovascular benefit; the evidence for each is limited.They do not feminize men: a pooling 32 studies found no effect of soy protein or isoflavones on testosterone or other reproductive hormones in men (Hamilton-Reeves 2010).East Asian populations have eaten soy all their lives, and the large body of safety data has not shown a problem.Concentrated isoflavone supplements: weaker data than for food, and unnecessary.
So: rather than chasing quercetin, fisetin or EGCG in supplements, make sure these 5-6 kinds of flavonoid sources are in your diet — the variety and combinations in food are something a single molecule cannot give.
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References · 5
- Xu, D., Hu, M.-J., Wang, Y.-Q., & Cui, Y.-L. (2019). Antioxidant activities of quercetin and its complexes for medicinal applications. Molecules, 24(6), 1123. 10.3390/molecules24061123
- Davis, J. M., Carlstedt, C. J., Chen, S., Carmichael, M. D., & Murphy, E. A. (2010). The dietary flavonoid quercetin increases VO(2max) and endurance capacity. International Journal of Sport Nutrition and Exercise Metabolism, 20(1), 56–62. 12 healthy untrained volunteers, quercetin 500 mg twice daily or placebo for 7 days, randomized double-blind crossover. VO2max rose 3.9% vs placebo and ride time to fatigue 13.2% (both p < .05). Year corrected from 2009 to 2010 (issue dated February 2010 per PubMed and Crossref); the id is kept (abstract, PMID 20190352). 10.1123/ijsnem.20.1.56
- Kressler, J., Millard-Stafford, M., & Warren, G. L. (2011). Quercetin and endurance exercise capacity: a systematic review and meta-analysis. Medicine & Science in Sports & Exercise, 43(12), 2396–2404. 10.1249/MSS.0b013e31822495a7
- Hickson, L. J., Langhi Prata, L. G. P., et al. (Kirkland, J. L. group). (2019). Senolytics decrease senescent cells in humans: preliminary report from a clinical trial of dasatinib plus quercetin in individuals with diabetic kidney disease. EBioMedicine, 47, 446–456. 10.1016/j.ebiom.2019.08.069
- Williamson, G., & Kerimi, A. (2020). Testing of natural products in clinical trials targeting the SARS-CoV-2 (Covid-19) main protease. Biochemical Pharmacology, 178, 114123. 10.1016/j.bcp.2020.114123