Capsaicin (8-methyl-N-vanillyl-6-nonenamide) is an active component of chilli peppers, which are plants belonging to the genus Capsicum. It is an irritant for mammals, including humans, and produces a sensation of burning in any tissue with which it comes into contact. Capsaicin and several related compounds are called capsaicinoids and are produced as secondary metabolites by chilli peppers, probably as deterrents against certain mammals and fungi. Pure capsaicin is a volatile, hydrophobic, colourless, odourless, crystalline to waxy compound.
Because of the burning sensation caused by capsaicin when it comes in contact with mucous membranes, it is commonly used in food products to give them added spice or “heat” (piquancy). In high concentrations, capsaicin will also cause a burning effect on other sensitive areas of skin. The degree of heat found within a food is often measured on the Scoville scale. In some cases, people enjoy the heat; there has long been a demand for capsaicin-spiced food and beverages. There are many cuisines and food products featuring capsaicin such as hot sauce, salsa, and beverages.
For information on treatment, see the section Treatment after exposure.
It is common for people to experience pleasurable and even euphoriant effects from ingesting capsaicin. Folklore among self-described “chilliheads” attributes this to pain-stimulated release of endorphins, a different mechanism from the local receptor overload that makes capsaicin effective as a topical analgesic. In support of this theory, there is some evidence that the effect can be blocked by naloxone and other compounds that compete for receptor sites with endorphins and opiates.
Capsaicin is used as an analgesic in topical ointments, nasal sprays (Sinol-M), and dermal patches to relieve pain, typically in concentrations between 0.025% and 0.25%. It may be applied in cream form for the temporary relief of minor aches and pains of muscles and joints associated with arthritis, backache, strains and sprains, often in compounds with other rubefacients. It is also used to reduce the symptoms of peripheral neuropathy such as post-herpetic neuralgia caused by shingles. In direct application the treatment area is typically numbed first with a topical anesthetic; capsaicin is then applied by a therapist wearing rubber gloves and a face mask. The capsaicin remains on the skin until the patient starts to feel the “heat”, at which point it is promptly removed. Capsaicin is also available in large bandages (plasters) that can be applied to the back.
Capsaicin creams are used to treat psoriasis as an effective way to reduce itching and inflammation.
The mechanism by which capsaicin’s analgesic and/or anti-inflammatory effects occurs is purportedly by mimicking a burning sensation; overwhelming the nerves by the calcium influx, and thereby rendering the nerves unable to report pain for an extended period of time. With chronic exposure to capsaicin, neurons are depleted of neurotransmitters, leading to reduction in sensation of pain and blockade of neurogenic inflammation. If capsaicin is removed, the neurons recover.
Capsaicin selectively binds to a protein known as TRPV1 that resides on the membranes of pain and heat-sensing neurons. TRPV1 is a heat-activated calcium channel that opens between 37 and 45 °C (98.6 and 113 °F, respectively). When capsaicin binds to TRPV1, it causes the channel to open below 37 °C (normal human body temperature), which is why capsaicin is linked to the sensation of heat. Prolonged activation of these neurons by capsaicin depletes presynaptic substance P, one of the body’s neurotransmitters for pain and heat. Neurons that do not contain TRPV1 are unaffected.
One study with human subjects indicates that capsaicin may be used to help regulate blood sugar levels by affecting carbohydrate breakdown after a meal.
Rodent studies have shown that capsicum may have some effectiveness against cancer. However, the American Cancer Society warns “available scientific research does not support claims for the effectiveness of capsicum or whole pepper supplements in preventing or curing cancer at this time”. Other uses not supported by evidence are: “addiction, malaria, yellow fever, heart disease, stroke, weight loss, poor appetite, and sexual potency”.
Capsaicin is the key ingredient in the experimental drug Adlea, which is in (as of 2007) ‘Phase 2 Trials’ as a long-acting analgesic to treat post-surgical and osteoarthritic pain for weeks to months after a single injection to the site of pain. Moreover, the drug purportedly reduces pain caused by osteoarthritis, joint and/or muscle pain from fibromyalgia and from other causes.
Capsaicin is also the active ingredient in riot control and personal defense pepper spray chemical agents. When the spray comes in contact with skin, especially eyes or mucous membranes, it is very painful, and breathing small particles of it as it disperses can cause breathing difficulty, which serves to discourage assailants. Refer to the Scoville Scale for a comparison of pepper spray to other sources of capsaicin.
Capsaicin is also used to deter pests, specifically mammalian pests. Targets of capsaicin repellants include deer, rabbits, squirrels, insects, and attacking dogs. Ground or crushed dried chilli pods may be used in birdseed to deter squirrels, taking advantage of the insensitivity of birds to capsaicin. The Elephant Pepper Development Trust claims the use of chilli peppers to improve crop security for rural African communities. Notably, an article published in the Journal of Environmental Science and Health in 2006 states that “Although hot chilli pepper extract is commonly used as a component of household and garden insect-repellent formulas, it is not clear that the capsaicinoid elements of the extract are responsible for its repellency.”
The first pesticide product using solely capsaicin as the active ingredient was registered with the U.S. Department of Agriculture in 1962. There are multiple manufacturers of a capsaicin-based gel product claiming to be a feral-pigeon (Columba livia) deterrent from specific roosting and loafing areas. Some of these products have an EPA label and NSF approval.
Capsaicin is a banned substance in equestrian sports because of its hypersensitising and pain-relieving properties. At the show jumping events of the 2008 Summer Olympics, four horses tested positive for the substance, which resulted in disqualification.
Acute health effects
Capsaicin is a highly irritant material requiring proper protective goggles, respirators, and proper hazardous material-handling procedures. Capsaicin takes effect upon skin contact (irritant, sensitiser), eye contact (irritant), ingestion, and inhalation (lung irritant, lung sensitiser). The LD50 in mice is 47.2 mg/kg.
Painful exposures to capsaicin-containing peppers are among the most common plant-related exposures presented to poison centres. They cause burning or stinging pain to the skin and, if ingested in large amounts by adults or small amounts by children, can produce nausea, vomiting, abdominal pain, and burning diarrhea. Eye exposure produces intense tearing, pain, conjunctivitis, and blepharospasm.
When used for weight loss in capsules, there has been a report of heart attack; this was thought to be due to excess sympathetic output.
The primary treatment is removal from exposure. Contaminated clothing should be removed and placed in airtight bags to prevent secondary exposure.
For external exposure, bathing the mucous membrane surfaces that have contacted capsaicin with oily compounds such as vegetable oil, paraffin oil, petroleum jelly (Vaseline), creams, or polyethylene glycol is the most effective way to attenuate the associated discomfort; since oil and capsaicin are both hydrophobic hydrocarbons the capsaicin that has not already been absorbed into tissues will be picked up into solution and easily removed. Capsaicin can also be washed off the skin using soap, shampoo, or other detergents. Plain water is ineffective at removing capsaicin, as are bleach, sodium metabisulfite and topical antacid suspensions. Capsaicin is soluble in alcohol, which can be used to clean contaminated items.
When capsaicin is ingested, cold milk is an effective way to relieve the burning sensation (due to caseins having a detergent effect on capsaicin); and room-temperature sugar solution (10%) at 20 °C (68 °F) is almost as effective. The burning sensation will slowly fade away over several hours if no actions are taken.
Burning and pain symptoms can also be relieved by cooling, such as from ice, cold water, cold bottles, cold surfaces, or a flow of air from wind or a fan. In severe cases, eye burn might be treated symptomatically with topical ophthalmic anesthetics, and mucous membrane burn with lidocaine gel. The gel from the aloe plant has also been shown to be very effective. Capsaicin-induced asthma might be treated with nebulized bronchodilators or oral antihistamines or corticosteroids.
Effects of dietary consumption
Ingestion of spicy food or ground jalapeño peppers does not cause mucosal erosions or other abnormalities. Some mucosal microbleeding has been found after eating red and black peppers, but there was no significant difference between aspirin (used as a control) and peppers. The question of whether chilli ingestion increases or decreases risk of stomach cancer is mixed: a study of Mexican patients found self-reported capsaicin intake levels associated with increased stomach cancer rates (and this is independent of infection with Helicobacter pylori) while a study of Italians suggests eating hot peppers regularly was protective against stomach cancer. Carcinogenic, co-carcinogenic, and anticarcinogenic effects of capsaicin have been reported in animal studies.
Effects on weight loss and regain
There is no evidence showing that weight loss is directly correlated with ingesting capsaicin, but there is a positive correlation between ingesting capsaicin and a decrease in weight regain. The effects of capsaicin are said to cause “a shift in substrate oxidation from carbohydrate to fat oxidation”. This leads to a decrease in appetite as well as a decrease in food intake. Even though ingestion of capsaicin causes thermogenesis, the increase in body temperature does not affect weight loss. However, both oral and gastrointestinal exposure to capsaicin increases satiety and reduces energy as well as fat intake. Oral exposure proves to yield stronger reduction suggesting that capsaicin has sensory effects. Short-term studies suggest that capsaicin aids in the decrease of weight regain. However, long-term studies are limited because of the pungency of capsaicin. Another recent study has suggested that the ingestion of capsaicinoids can increase energy expenditure and fat oxidation through the activation of brown adipose tissue (BAT) in humans from the effects of the capsaicin.