Taking hay fever pills may put men at higher risk of infertility, a study claims.
The antihistamine tablets, which cost around $10 each, treat symptoms of mild allergies such as hay fever.
They control levels of histamine, a molecule produced by the immune system when facing a threat.
Scientists in Argentina, who mostly tested animals, concluded hay fever tablets are likely to affect the production of hormones in the testicles.
This can hit sex drive, lead to lower-quality sperm and less. About one in six UK couples struggle to conceive.
Up to a third of Brits suffer from hay fever.
Antihistamines are also taken for hives, conjunctivitis and reactions to insect bites.
The University of Kents Darren Griffin said their use is on the rise, and men might want to avoid taking too many if they are trying to start a family. But he added: Given the data is mostly in animals the jury is still out.
Newcastle Universitys Dr Richard Quinton said: The findings arent likely to be relevant to anti-histamines we take for allergies.
The Proprietary Association of Great Britain, which represents the manufacturers of over-the counter medicines, said users need not worry.