One of the world's most common insect repellents acts on the central nervous system in the same way as some insecticides and nerve gases, according to a study released on Wednesday.
Moderate use of the chemical compound, called deet, is most likely safe, the researchers say.
But experiments on insects, as well as on enzymes extracted from mice and human neurons, showed for the first time that it can interfere with the proper functioning of the nervous system.
The researchers say further studies are "urgently needed" to assess deet's potential toxicity to humans, especially when combined with other chemical compounds.
Their findings may also shed some light on the so-called "Gulf War Syndrome," the name given to a complex and variable mix of neurological symptoms reported by tens of thousands of US military veterans who served in the first Gulf War against Iraq in 1990-1991.
Developed by US Department of Agriculture scientists just after World War II, deet has been available as a bug repellent for more than five decades.
Sold as lotions, creams and sprays in concentrations from five to 100 percent, it has been widely used not just by weekend campers but as a frontline barrier against malaria, dengue fever and other mosquito-borne diseases.
Some 200 million people use deet-based products every years, according to the study, published in the British-based open-access journal BMC Biology.
Scientists still don't know exactly how the compound works on blood-seeking insects. Some say it blocks the sensory neurons that would be titillated by a potential meal, while others hypothesise that bugs are simply put off by the smell.
More surprising still, there is relatively little research on the effects of deet in humans.
"It has been used for many years, but there are recent studies now that show a potential toxicity," said Vincent Corbel, a researcher at the Institute for Development Research in Montpellier, France, and lead author of the study.
"What we have done is identify a neurological target for this compound," he told AFP by phone.
In experiments, Corbel and a team of scientists co-led by Bruno Lapied of the University of Angers discovered that deet interferes with the normal breaking down of acetylcholine (ACh), the most common neurotransmitter in the central nervous system.
It does so by blocking the enzyme that normally degrades ACh, acetylcholinesterase, or AChE. The result is a toxic build-up of ACh that ultimately prevents the transmission of signals across the neuron synapse, the study found.
A class of insecticides called carbamates, as well as the nerve gas sarin, work in the same way, only the effects are stronger and last much longer.
Which is where the Gulf War Syndrome comes in.
"Many of the pesticides used in the Gulf War, as well as PB and nerve agents, exert toxic effects on the brain and nervous system by altering levels of ACh," a US government report issued last November concluded.
PB, or pyridostigmine bromide, was widely used to protect against nerve gas exposure.
The 450-page report, entitled "Gulf War Illness and the Health of Gulf War Veterans," points to earlier evidence that overexposure to deet may be toxic for the nervous system, but fails to recognize its potential role as a booster for the more potent chemicals to which soldiers had been exposed.
"For US soldiers, the cocktail of high doses of PB and insect repellents to protect against mosquito bites may have caused symptoms, as both act on the central nervous system in the same way," said Corbel.
Fortunately, deet is "reversible," meaning its impact is short-lived. But further studies are needed to determine at what concentration it may become dangerous to people, especially small children and pregnant women, he added.