CHICAGO — Capsaicin, a chemical that gives cayenne peppers their fiery sting, can chill out persistent pain following shingles and may someday help ease chronic pain from diabetes or surgery, a study shows.
Dr. Mark Dahl, professor of dermatology at the University of Minnesota Medical School, said the finding is not as strange as it sounds.
He said the reason capsaicin in peppers burns the mouth is that it causes cells to release something called "substance-P," which tells nerves to send a pain signal to the brain.
But in the cream applied to areas aching from post-shingles pain, capsaicin "depletes substance-P from the skin, so there is none left to tell the brain you have pain from zoster," the herpes virus that causes shingles, Dahl said in an interview.
Dahl recently studied 32 people who had suffered for more than a year with excruciating, untreatable post-shingles pain. Shingles is a severe infection that causes painful blisters in the skin above underlying nerves infected by the herpes virus. After the blisters have cleared up, about 20% of shingles patients experience the chronic post-shingles pain, which can linger for years.
Triple-strength prescription capsaicin cream cut pain more than 40% in slightly more than half the patients, a significant result for patients who had no other remedy for their aching.
A dilute form of the cream, called Zostrix, is sold over the counter, and Dr. Joel Bernstein, head of GenDerm Corp., the Northbrook, Ill., company that makes the drug, said many patients find this less expensive ointment sufficient. He said several hundred patients have tested capsaicin, with 70% to 80% getting at least some pain relief from it.
Dahl said capsaicin "can sting like crazy on normal skin, but on these areas with shingles" pain is diminished. The drug takes several weeks to have its pain-blunting effect, and is not for treating short-term, acute pain.
About 300,000 people in the United States each year suffer from post-shingles pain and Dahl said people over 55 who contract shingles are most likely to suffer the lingering agony.
"It's very bad, like a toothache," Dahl said of post-shingles pain. "Incredibly annoying, distressing and depressing."
Bernstein said post-shingles pain is a major cause of suicide in the elderly, and noted pain-killing medications now used to try to combat it can be addictive or have serious side effects and interactions with other drugs.
Bernstein's firm is trying to find a way of producing capsaicin cheaply in the laboratory. Currently, capsaicin is extracted from peppers from Europe, the Orient and Michigan, with 10,000 pounds of peppers needed to make one pound of pure extract.
Helpful in Mastectomy
In a study to be published soon in the journal Pain, Bernstein will describe other new tests of capsaicin's effects in women suffering pain long after a mastectomy.
He said some people, months or years after amputation and other surgery, develop benign tumors, or neuromas, at the ends of nerves that have been severed. The neuromas can be excruciating.
The company is also studying whether capsaicin can treat pain associated with diabetes. Sometimes called "burning foot syndrome," the pain "can make feet, toes or ankles burn, or make them exquisitely sensitive to touch, even wearing shoes or socks," Bernstein said.
The company has applied to the Food and Drug Administration for approval of the high-strength prescription drug for post-shingles pain.