Prolonged or excessive use of high-concentration insect repellents should be avoided because it can cause allergic and toxic reactions, The Medical Letter reports.

The most effective insect repellent applied to the skin is the chemical diethyltoluamide (N,N-diethyl-m-toluamide), commonly (and thankfully) known as "deet." It is effective against mosquitoes, chiggers, ticks, fleas and biting flies.

But medical journals report a growing number of serious human reactions to deet, which is absorbed through the skin. Even in low concentrations, it can cause hives and skin rashes.

The Medical Letter, a newsletter covering the pharmaceutical industry, cited two reports of more serious effects. A woman went into anaphylactic shock after touching someone who had just applied a 52 percent solution of deet repellent, and an 8-year-old girl developed a rash and altered behavior after a few days' use of 15 percent deet and then had a grand mal seizure within hours of her first use of another repellent containing almost 100 percent deet.

The longterm effects of deet, the newsletter says, are still unknown.

Some widely available liquid repellents -- including Jungle Plus, Muskol and Ben's 100 -- contain 100 percent deet.

In the early days of its use, before the 1970s, deet caused toxic and allergic reactions mainly after excessive or prolonged use by children. But with the growing availability of high-concentration repellents, warns the Letter, "brief exposure to smaller amounts has caused serious reactions in children and adults."

The newsletter's conclusion: Repellents with lower concentrations of deet may have to be applied more often, "but are usually less oily and cosmetically more acceptable, and they may be safer."