It's the most popular cosmetic procedure in America -- injecting Botox into one's forehead and around the eyes to smooth the lines of aging.
But some patients report a more profound benefit: Their disabling migraines suddenly stop.
"My everyday headaches were gone," said 37-year-old Martha Fritz. "I had no more migraines. [I] was like a new person."
Fritz had suffered headaches most of her life. A dozen medications could not stop the pain.
"Each time it would go away for maybe a half hour," she said. "It would come right back. Nothing worked."
The pain stopped after she entered a headache study and got Botox injections every few months.
"I was in the bathroom getting ready for work when it hit me: I didn't have a headache," she said.
When the results from all the cases were added up, however, the study found Botox was not any better than a placebo.
That discovery left many doctors puzzled. Clearly some patients got dramatic relief with those injections. But how? Now, researchers say it may depend on the type of migraine people suffer.
"It was surprising, frankly, to hear, but the numbers are very suggestive. There may be something to the character of the pain," said Dr. Thomas Ward of Dartmouth Medical Center.
Specifically, researchers found that Botox was 80 percent to 90 percent effective in radiating pain in the head, so-called "imploding headaches," where the pain feels as though it's moving from the outside to the inside of the brain. Patients report a "squeezing" or "crushing" sensation on the scalp, as though they're caught in a vise.
"It may be the lining of the brain is particularly involved in the process here," Ward said. "Botox may actually prevent the release of substances that can initiate and drive the pain."
It's estimated that about a third of migraine sufferers have these "imploding headaches." Fritz did until she got Botox.
"You can concentrate better. You can enjoy your family better," Fritz said after her headaches disappeared. "It has given me the quality of life that I always wanted."
ABC News' John McKenzie filed this report for "World News Tonight."