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What people mean by shanghuo
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In one pass What people call shanghuo (literally, rising fire) is often one word for several different complaints: an ulcer inside the mouth, a sore throat, gums that bleed when brushed, small blisters at the edge of the lip, spots…
Educational content, not medical advice. See a clinician about your own case.
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Chapter 1
One word, several separate events
What people call shanghuo (literally, rising fire) is often one word for several different complaints: an ulcer inside the mouth, a sore throat, gums that bleed when brushed, small blisters at the edge of the lip, spots on the face, several days without being able to pass stool, a nosebleed. They happen in different tissues and have different causes; what they share is only that they turn red, swell or hurt, or leave a person feeling hot and dry.
The fiery burn in the mouth after spicy food is often counted as shanghuo too. In fact the burn is a nerve signal: the capsaicin in chili opens a pain channel on sensory nerves called TRPV1, and the tissue in the mouth has not been heated.
This piece only takes these symptoms back, one at a time, to the mechanisms in the body, and does not judge the theories of traditional medicine. One of them needs particular attention: a sore throat so bad that breathing is hard or saliva cannot be swallowed needs medical care right away.
The fiery burn in the mouth after spicy food is often counted as shanghuo too. In fact the burn is a nerve signal: the capsaicin in chili opens a pain channel on sensory nerves called TRPV1, and the tissue in the mouth has not been heated.
This piece only takes these symptoms back, one at a time, to the mechanisms in the body, and does not judge the theories of traditional medicine. One of them needs particular attention: a sore throat so bad that breathing is hard or saliva cannot be swallowed needs medical care right away.
Mechanism · Spicy heat is a nerve signal
A 1997 study established that the molecule capsaicin acts on is TRPV1. It is an ion channel that heat can open, and it is the part of the pain system that reports scalding heat in the first place. Capsaicin and something scalding open the same channel, so the brain reads both as burning.What else capsaicin does in the body is covered in more detail in the piece on chili peppers.
Background · Where the others happen
Sore throats, bleeding gums and blisters at the lip each get their own scene later. The rest are not expanded in this piece; put back into the body, most of them have a place of their own:Mouth ulcers: on the mucosa inside the mouth. Common triggers include accidental bites, a sharp tooth, burns, tiredness, stress, anxiety, hormonal changes and a lack of vitamins or minerals; they usually heal on their own in one to two weeks.Spots: dermatologists call this acne vulgaris, a problem of the skin itself, and it has its own clinical guideline.Constipation: happens in the colon and has to do with how the gut moves.Nosebleeds: dry air and winter heating dry out the lining of the nose, and the blood vessels in it break more easily.
The immune mechanism of mouth ulcers, acne, constipation and the nose each have their own pieces that go into detail. Whether a particular food is especially likely to bring on shanghuo is not discussed here.
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Chapter 2
Lip blisters and cracked corners
A cluster of small blisters at the edge of the lip is often a cold sore, caused by herpes simplex virus type 1 (HSV-1). Many people are infected in childhood, and the virus does not leave afterwards: it hides and lies dormant in the sensory nerve cells of the trigeminal ganglion. When something like a fever, sun exposure or stress comes along, it becomes active again and flares up on the lip once more: first tingling, itching and burning, then blisters.
Red, moist, cracked and crusted corners of the mouth are a different matter, called angular cheilitis. Its local causes fall into irritant, allergic and infectious groups, which can act alone or stack together; nutritional factors, whole-body illnesses and medicines can also cause it, on their own or together with local factors.
Red, moist, cracked and crusted corners of the mouth are a different matter, called angular cheilitis. Its local causes fall into irritant, allergic and infectious groups, which can act alone or stack together; nutritional factors, whole-body illnesses and medicines can also cause it, on their own or together with local factors.
Numbers · Most people carry this virus
The World Health Organization (WHO) estimated in 2025 that about 3.8 billion people under 50 worldwide have HSV-1 infection, about 64%. The virus lives in nerve cells and switches back and forth between resting and active; medicines can treat a flare-up but cannot clear the virus.It spreads mainly through contact with sores around the mouth, saliva and the skin around the mouth. Skin that looks normal can also pass it on. Infection usually happens in childhood, through close contact such as kissing.
Mechanism · From tingle to scab
A flare-up follows a set order. First a spot on the lip tingles, itches or burns; within about 48 hours painful small blisters appear; then they burst and scab over, and usually start to heal within 10 days. The whole stretch, from the first tingle until the sore has completely healed, is contagious.Even with no symptoms at all, saliva can carry the virus. In people with weakened immunity, flare-ups tend to last longer and be more severe and more painful; the median healing time in the literature can reach about 28 days.
Evidence · What wakes the virus up
A 2023 narrative review collected the triggers that are commonly reported: stress, fever, ultraviolet light, menstruation and hormonal changes, dental work or surgery, injury, medicines such as corticosteroids, immunosuppression, plus tiredness, lack of sleep, chapped lips and other viral infections. It is a compiled list, and the evidence behind each item varies in strength.Ultraviolet light has been tested directly in one experiment. In a small 1985 experiment, 5 patients with a history of recurrences had a sunlamp shone on the spot where their sores usually appear; 6 of 10 exposures set off a cold sore, and 3 of the 4 highest-dose exposures produced large blisters that were positive on virus culture. With only 5 people, the result shows only that ultraviolet light can trigger a flare-up at the usual spot; it cannot be used to estimate the risk for people in general.
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Chapter 3
Sore throat: mostly a virus in the pharynx
The most common cause of an acute sore throat is a virus. The virus infects the mucosa of the pharynx and causes acute pharyngitis, an acute inflammation of the pharynx, and so the throat starts to hurt. The most common bacterial cause is group A Streptococcus, but it accounts for only a minority, and fewer in adults than in children.
Most sore throats get better on their own in about a week.
Most sore throats get better on their own in about a week.
Numbers · Strep is the minority
The 2012 guideline from the Infectious Diseases Society of America (IDSA) gives these shares: among people who see a doctor for a sore throat, group A Streptococcus accounts for about 5%–15% of adults and 20%–30% of children. Most of the rest are viruses.These figures describe the share among people who come in for care, not the chance that any one person has strep throat.
Clinical · Signs that point to a virus
The guideline lists two sets of clues. Clear viral features such as cough, runny nose, hoarseness and small ulcers in the mouth strongly suggest a virus rather than strep. Strep throat, by contrast, often starts suddenly, with pain on swallowing, fever, and swollen lymph nodes at the front of the neck that hurt when pressed.These clues cannot replace a doctor's judgement.
Evidence · Most are pain-free by one week
A 2021 Cochrane systematic review pooled 29 trials and 15,337 cases of sore throat. In the groups given only a placebo or no treatment, 82% had no symptoms by one week.The figures have clear limits: most of the trials were done in the 1950s, and few separated bacteria from viruses. What they support is a plain conclusion: most sore throats settle down on their own.
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Chapter 4
Bleeding gums: plaque at the gum line
Gums that bleed when brushed are a common sign of gingivitis. Where tooth meets gum, bacteria build up into a sticky layer called plaque. If brushing and cleaning between the teeth do not remove it, it keeps irritating the gum: the gum turns red, swells and gets sore, and bleeds during brushing, flossing or eating hard foods.
Plaque is a bacterial biofilm. The inflammation it causes stays confined to the gum, and once the biofilm is removed or broken up, the inflammation can reverse. How severe the inflammation gets does not depend on plaque alone: hormones, whole-body illnesses and medicines all change how mild or severe it is.
Plaque is a bacterial biofilm. The inflammation it causes stays confined to the gum, and once the biofilm is removed or broken up, the inflammation can reverse. How severe the inflammation gets does not depend on plaque alone: hormones, whole-body illnesses and medicines all change how mild or severe it is.
Evidence · When cleaning stops
In the 1965 study of experimental gingivitis, dental students with healthy gums stopped cleaning their teeth; plaque built up and mild gingivitis appeared, and once cleaning resumed the inflammation subsided.The classification consensus paper adds one point: setting off gingivitis takes a fairly heavy build-up of plaque.
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Numbers · How much bleeding counts
The usual measure a dentist uses to judge gingivitis is bleeding on probing: a probe is pressed gently at each site along the gum margin to see how many of them bleed. A 2018 consensus case definition holds that this is a simple, objective measure.Under this definition, a person whose periodontal tissues are intact counts as a gingivitis case when 10% or more of sites bleed; 10%–30% is called localized and more than 30% generalized. These cut-offs come from the 2018 consensus definition; and one bleeding site does not make the person a gingivitis case.
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Clinical · Gingivitis and periodontitis
Gingivitis itself can be reversed, but it is linked to deeper trouble. The consensus paper states that controlling gum inflammation is key to preventing periodontitis.How periodontitis breaks down the tissues that hold the teeth in place is covered in detail in its own piece on the gums.
Chapter 5
Name the symptom first, then find its cause
Taking shanghuo apart, three questions can be asked in order.
Where is it: on the mucosa inside the mouth, on the edge of the lip, at the corner of the mouth, in the pharynx, at the gum line, or inside the nostril. What does it look like: a shallow little crater, a cluster of blisters, a crack, redness and swelling, or bleeding. How long has it lasted: how many days, and does it keep coming back.
Asking where is not always enough: a cluster of blisters on the edge of the lip and a crack at the corner of the mouth sit close together but have different causes; the first is the herpes simplex virus, the second is angular cheilitis.
These three questions only help put the symptom into words; they cannot replace a diagnosis. When in doubt, or when a red-flag sign appears, a doctor or dentist should take a look.
Where is it: on the mucosa inside the mouth, on the edge of the lip, at the corner of the mouth, in the pharynx, at the gum line, or inside the nostril. What does it look like: a shallow little crater, a cluster of blisters, a crack, redness and swelling, or bleeding. How long has it lasted: how many days, and does it keep coming back.
Asking where is not always enough: a cluster of blisters on the edge of the lip and a crack at the corner of the mouth sit close together but have different causes; the first is the herpes simplex virus, the second is angular cheilitis.
These three questions only help put the symptom into words; they cannot replace a diagnosis. When in doubt, or when a red-flag sign appears, a doctor or dentist should take a look.
Red flag · When a sore throat is an emergency
The NHS sore throat page sorts the warning signs into three levels.Emergency care right away: difficulty breathing, or being unable to swallow at all; drooling, which means even saliva cannot be swallowed; a high-pitched sound when breathing in, called stridor; symptoms that are severe and getting worse quickly.Prompt medical care: a very high temperature, or feeling hot and cold and shivery; signs of dehydration; a weakened immune system.See a doctor: no improvement after a week; frequent sore throats; a lump felt in the mouth or neck.
Red flag · Sores in and around the mouth
A mouth ulcer that has not healed after 3 weeks needs a doctor or dentist; so does one that is unlike earlier ones, bleeds, or keeps getting redder and more painful.A cold sore needs a doctor in these cases: it has not started to heal within 10 days; it is very large or very painful; the gums are red, swollen and sore with ulcers in the mouth at the same time; the person has a weakened immune system.
Red flag · Gums, nosebleeds and constipation
Very sore and swollen gums, loose teeth, ulcers or red patches in the mouth, or a lump in the mouth or on the lip call for a dentist appointment soon.How to pinch the nose during a nosebleed, for how long, and when to go to the emergency department are listed in full in the piece on nasal breathing. Constipation has to do with how the gut moves, and when it needs a doctor is covered in the piece on constipation.
These signs mostly come from guidance that health bodies write for patients. Their job is to signal when to see a doctor, not to let people diagnose themselves.
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References · 11
- NHS. (2024). Mouth ulcers. www.nhs.uk/conditions/mouth-ulcers
- NHS. (2024). Sore throat (page last reviewed 8 April 2024). www.nhs.uk/conditions/sore-throat
- NHS. (2026). Gum disease (page last reviewed 20 April 2026). www.nhs.uk/conditions/gum-disease
- NHS. (2024, February 19). Cold sores. NHS website. www.nhs.uk/conditions/cold-sores
- Caterina, M. J., Schumacher, M. A., Tominaga, M., Rosen, T. A., Levine, J. D., & Julius, D. (1997). The capsaicin receptor: a heat-activated ion channel in the pain pathway. Nature, 389(6653), 816-824. 10.1038/39807
- Gopinath, D., Koe, K. H., Maharajan, M. K., & Panda, S. (2023). A comprehensive overview of epidemiology, pathogenesis and the management of herpes labialis. Viruses, 15(1), 225. 10.3390/v15010225
- World Health Organization. (2025, May 30). Herpes simplex virus [Fact sheet]. www.who.int/news-room/fact-sheets/detail/herpes-simplex-virus
- Park, K. K., Brodell, R. T., & Helms, S. E. (2011). Angular cheilitis, part 1: Local etiologies. Cutis, 87(6), 289-295. pubmed.ncbi.nlm.nih.gov/21838086
- Park, K. K., Brodell, R. T., & Helms, S. E. (2011). Angular cheilitis, part 2: Nutritional, systemic, and drug-related causes and treatment. Cutis, 88(1), 27-32. pubmed.ncbi.nlm.nih.gov/21877503
- Shulman, S. T., Bisno, A. L., Clegg, H. W., Gerber, M. A., Kaplan, E. L., Lee, G., Martin, J. M., & Van Beneden, C. (2012). Clinical practice guideline for the diagnosis and management of group A streptococcal pharyngitis: 2012 update by the Infectious Diseases Society of America. Clinical Infectious Diseases, 55(10), e86-e102. 10.1093/cid/cis629
- Murakami, S., Mealey, B. L., Mariotti, A., & Chapple, I. L. C. (2018). Dental plaque-induced gingival conditions. Journal of Periodontology, 89(Suppl 1), S17-S27. 10.1002/JPER.17-0095
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