DEET, which is the the active ingredient used by brands of insect repellent including Off, 3M and Cutter has been proven to be ineffective in fending off certain kinds of mosquitoes.

The London-based study showed that a certain kind of mosquito found in tropic regions that spreads diseases like yellow fever was not repelled by DEET after it was exposed to it a second time. However, it is not yet known how long it takes for the repellent to become ineffective.

“It’s quite a worrying find,” says the BBC’s Science Correspondent Rebecca Morelle. Morelle says more research will be needed to gauge the effect of the findings on regular bug repellent use.

Scientists say that although DEET has been proven to be less effective in some cases, it should still be used when a person is exposed to insects.