Insect repellents may be essential for summer's warm weather, but overuse of some of the most popular of these products is creating a buzz among pharmacologists.

Each year, roughly a third of Americans use some product containing DEET, the active ingredient for almost all insect repellents. Although the chemical is safe for the vast majority of users, researchers warn that using it can cause brain damage in rare cases.

In particular, they caution that consumers should avoid using such products in large amounts or in high concentrations for extended periods of time or in combination with prescription drugs.

Mohamed Abou Donia, professor of pharmacology and cancer biology, has experimented with DEET and found that rats, when given large amounts of the repellent, develop serious injury to certain parts of the brain.

"We found that the explanation of the damage is that there was nervous cell death [in the area] that controls movement, caused by exposure to DEET," Abou Donia said. The areas damaged were the cortex, cerebellum and hippocampus, causing problems in muscle control and performance of basic functions.

He said such problems are very rare among humans but that people can avoid them even more by using DEET products for only a week at a time and not concurrent with prescription medications. In addition, Abou Donia said using a repellent with over a 30 percent concentration of DEET can also put a bug bite target at risk of greater disorders.

Products containing DEET are labeled with such information.

Other researchers noted that serious treatment is required for humans who have been exposed to DEET in these unadvisable ways.

"DEET by itself is probably one of the more harmless chemicals we deal with. The problem comes when DEET interacts with other chemicals," said Ernest Hodgson, professor of toxicology at North Carolina State University. "We've seen at least some mechanisms by which you might get very serious interactions."

Whether people can safely mix prescriptions with DEET must be decided on an individual basis, he said.

Hodgson also noted that products containing DEET dominate the insect repellent market and that even though such products have the potential for serious danger in some cases, people should use them when outside in high-insect areas.

Short for N, N-diethyl-m-toluamide, DEET has been around since the 1940s. In 1996, Medical Center researchers released a study linking DEET to the Persian Gulf War Syndrome, because the repellent was used by U.S. troops in combination with several other chemicals during 1990s military operations in the Middle East.