The popular over-the-counter painkiller ibuprofen widely used for aches and pains could make young men infertile by shrinking their testicles, scientists warn.
The drug that costs just 2p for a 200mg tablet lowers the male sex hormone testosterone.
The drop in the hormones produced by the testicles then causes them to shrivel up causing fertility problems.
A study of 18 to 35 year-olds found the painkiller increased the risk of the disorder compensated hypogonadism, which usually only affects the elderly or smokers.
Overuse of ibuprofen could also lead to full-blown hypogonadism- or low testosterone levels - which has been linked to premature death.
Dr David Kristensen, of Copenhagen University, said: “Through a clinical trial with young men exposed to ibuprofen, we show that the analgesic resulted in the clinical condition named ‘compensated hypogonadism’ - a condition prevalent among elderly men and associated with reproductive and physical disorders.”
It occurs when men have normal levels of testosterone but higher levels of luteinizing hormone (LH) - a chemical that stimulates the production of testosterone.
Dr Kristensen said: “In the men, luteinizing hormone (LH) and ibuprofen plasma levels were positively correlated, and the testosterone/LH ratio decreased.”
The study of 31 male participants found the administration of ibuprofen reduced production of testosterone by nearly a quarter in about six weeks - resulting in compensated hypogonadism.
In experiments on testicles from prostate cancer donors and cultured testes cells, the researchers found ibuprofen induced the condition by affecting hormones.
Dr Kristensen said: “Concern has been raised over increased male reproductive disorders in the Western world and the disruption of male hormones has been suggested to play a central role.
“Several studies have shown mild analgesics exposure during foetal life is associated with anti-androgenic effects and congenital malformations.
“But the effects on the adult man remain largely unknown.”
Dr Kristensen said said painkillers such as paracetamol, aspirin and ibuprofen are among the most commonly drugs worldwide.
Ibuprofen is “especially interesting” because of its increasing use in the general population - and by elite athletes - for aches, pains, fever and arthritis.
In the first study of its kind the researchers split participants into two groups - with 14 receiving 600mg of ibuprofen twice a day two weeks before and 30 days after an exercise session. The others acted as a control.
After 14 days blood tests showed there was an 18% decrease in testosterone - rising to 23% after 44 days.
The experiments on testes from human donors and cells grown in the lab then revealed the mechanism, reports Proceedings of the National Academy of Sciences.
Dr Kristensen said: “Taken together these data suggest that ibuprofen induced a state of compensated hypogonadism - which occurred as early as 14 days and was maintained until the end of the trial at 44 days.”
He said investigating ibuprofen-induced compensatory hypogonadism is crucial as the disorder is “generally associated with smoking and ageing.”
Dr Kristensen said: “Moreover, compensated hypogonadic men present with an increased likelihood of reproductive, cognitive and physical symptoms.
“Further characterisations of the state of compensated hypogonadism induced by ibuprofen - which was already established after 14 days of ibuprofen administration - are therefore important in determining the potential effects on healthy young men.”
He added: “Moreover, ibuprofen appears to be the preferred pharmaceutical analgesic for long-term chronic pain and arthritis.
“Therefore it is also of concern that men with compensated hypogonadism may eventually progress to overt primary hypogonadism, which is characterised by low circulating testosterone and prevalent symptoms including reduced libido, reduced muscle mass and strength, and depressed mood and fatigue.”
Previous research has found an association between low testosterone levels and premature death from heart disease and all causes.
A study of almost 1,000 men tracked for seven years found almost twice as many with testosterone deficiency - or hypogonadism - died as did those with normal levels.
Compensated hypogonadism occurs before any obvious sypmtoms develop.