Capsaicin (Hot Pepper Extract) for Neuropathic Pain
Capsaicin (Hot Pepper Extract) for Neuropathic Pain
January 1998; Volume 1: 7-9
By David Schiedermayer, MD, FACP
Many chronic conditions present physicians with the challenge of intractable pain. In a search to make patients more comfortable when pain results from conditions such as diabetic neuropathy or post-herpetic neuralgia, physcians may want to recommend one of the many creams that incorporate an extract of hot peppers. Since winning approval from the federal U.S. F.D.A. as an external analgesic, capsaicin has become a popular ingredient in prescription and over-the-counter analgesic creams. Capsaicin creams can provide clinicians with an easily available, safe alternative for some patients.Introduction
Capsaicin is the active ingredient in an extract of hot peppers. It
is what makes hot peppers hot and is most concentrated in the rib or membrane
of the
pepper, which attaches its seeds to its flesh. Seeds are second in
concentration of capsaicin and the flesh third. There is some general truth
to the axiom "the smaller the pepper, the greater its heat," but like most
axioms, there are many exceptions. Habañero peppers have the most
heat; bell peppers have little or none.
Indications
Creams made with the plant extract capsaicin are indicated for a number of conditions, including:
• post-herpetic neuralgia
• post-mastectomy pain
• hemodialysis-associated pruritus
• psoriatic itching and pain
• painful neuropathies, especially diabetic neuropathy
• other superficial neuropathies
Tradition
Chili peppers are grown worldwide and have been for thousands of years. Culinary cayenne pepper purchased in the local grocery story is usually an amalgamation of whatever dried, pungent capsicums the manufacturer has on hand. Capsaicin for medicinal use comes from a species of the cayenne pepper, Capsicum frutescens.
Culture
Chili peppers have been compounded into a pain-relieving salve for centuries in Africa. The actual philosophical underpinning of capsaicin is that it is a "counter-irritant"—an old concept in which a substance is placed on a painful area, causing irritation, which actually helps relieve the pain. Only recently has the scientific underpinning of this concept been described with the effect of capsaicin on substance P. In this sense, capsaicin is only one of a number of these substances (mustard plasters, etc.) which cause irritation—in a sense, one of the new elements in the "counter-irritant" armamentarium.
Contrary to folklore, capsaicin does not stimulate circulation or cause intestinal or gastric ulceration. Indeed, some anecdotal evidence suggests a protective effect on the intestinal mucosa.
Pharmacokinetics
Capsaicin is not water soluble and, in animals, is metabolized by the liver.
Mechanisms of Action
In specific concentration, capsaicin depletes substance P in afferent type C sensory nerve fibers and affects only proprioception. Paradoxically, proprioception may be heightened by incomplete depletion of substance P. Capsaicin has no effects on other afferent sensations, such as touch and vibration. Unlike other treatments for neuropathy, such as local anesthetics, opiates, anti-seizure medications, or tricylic antidepressants, capsaicin specifically treats pain without impairing other aspects of the nervous system.
Clinical Studies
An excellent monograph by James Utt is available in the DrugDex through the Internet.1 Utt lists 33 references, most of which are related to trials of capsaicin in the clinical conditions for which it is indicated, listed above. The most important clinical study is by Bernstein et al, who performed a randomized, double-blind clinical trial of capsaicin cream for the treatment of post-herpetic neuralgia.2 The trial used the high potency form of the cream (0.075%) in 32 patients ages 54-90. Clinical improvement was reported in 77% and 31% of the patients treated with capsaicin and placebo, respectively. The visual analog scale for pain relief showed 40% relief of pain in 54% of capsaicin patients vs. 6% in placebo patients, after six weeks of treatment. Burning, stinging, or redness were noted in five patients on capsaicin and two on placebo. Another study by Drake et al, however, did not demonstrate this degree of efficacy.3 Capsaicin has also been shown to cause a reduction in itching and redness in psoriasis.4
Adverse Effects
The cream should not be applied to open wounds or abrasions. Care should be taken to wear gloves during and wash hands after its application and to avoid contact with the eyes and mucous membranes. The cream must not be applied to open areas of skin, and patients with shingles and psoriasis especially should be monitored for skin changes. Capsaicin cream can cause severe mucous membrane burning and irritation, and even corneal abrasions. In addition, concentrations of capsaicin greater than 0.1% have been found to have potential respiratory toxicity, and there are reports of local chemical irritation, paroxysms of cough, and bronchospasm when aerosolized capsaicin is inhaled.
Drug Interactions
Potential interaction with an angiotensin converting enzyme (ACE) inhibitor has been reported in one patient, who developed a persistent cough after applying capsaicin cream 0.075%. The patient had been taking the ACE inhibitor for several years without cough.
Formulation
Like many therapeutic plant substances, capsaicin is a complex base and is derived directly from the seeds and plant membranes. Capsaicin is also available as fresh and dried peppers, capsules, tablets, and tinctures.
Some individuals may gather fresh chilies, then dry and powder them in a coffee grinder or blender to create a liniment for alleviating pain. The safety and efficacy of these and other home preparations in alleviating pain is dubious at best, as the concentration of capsaicin cannot be regulated and may easily be aerosolized, causing transient respiratory discomfort. When applied, irritation and burning may result. Home liniments using cayenne should be discouraged.
Application
The proper method of application is to apply the cream 4-5 times daily for at least four weeks before assessing effectiveness in an individual patient. Some writers have recommended more frequent application (up to 10 times daily), but this is not recommended by the manufacturers. Application less frequently than recommended, such as once or twice daily, can actually lead to increased pain.
Course
Several weeks of therapy are required for maximal effect. In patients whose pain is short-lived, the medication can be discontinued after several months and the pain re-evaluated. Older patients, especially those with longstanding post-herpetic neuralgia, may require several years of therapy and may even need lifelong treatment.
Dosage
Commercially available forms of capsaicin are white, unguent-like creams that vary in potency from 0.01% to 0.075%. Over-the-counter preparations are available and reasonably priced.
Conclusions
Capsaicin is an excellent addition to the physician’s pharmacologic treatment arsenal for specific and nonspecific neuropathic pain. It also appears to alleviate itching in some patients with dialysis-mediated pruritus and in psoriatic patients. It is fairly low risk and noninvasive. Patients can safely self-modulate the frequency and length of use within the parameters described above.
Main Points
Contact with the eyes, nose, and mouth must be avoided. The cream must be applied more than twice daily for several weeks to be effective. v
References
1. Utt J. Drug Evaluation Monographs, DrugDex, Micromedex; 1991, 1995.
2. Bernstein JE, et al. Topical capsaicin treatment of chronic postherpetic neuralgia. J Am Acad Dermatol 989;21:265-270.
3. Drake HF, et al. Randomized double-blind study of topical capsaicin for treatment of post-herpetic neuralgia. Pain 990;5:111.
4. Bernstein JE, et al. Effects of topically applied capsaicin on moderate and severe psoriasis vulgaris. J Am Acad Dermatol 986;15:504-507.
Dr. Schiedermayer is Associate Professor of Medicine, Medical College of Wisconsin, Milwaukee.
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