Doctors are warning men to be careful when taking painkillers for weeks at a time, saying it could affect their fertility.
A new study found taking just three ibuprofen tablets a day for more than a couple of weeks impairs the body's ability to produce testosterone.
The more ibuprofen the subjects took, the bigger the negative effect the researchers found.
"Ibuprofen use results in... repression of endocrine cells in the human testis," the study found.
"This repression results in the elevation of the stimulatory pituitary hormones, resulting in a state of compensated hypogonadism - a disorder associated with adverse reproductive and physical health disorders."
Ibuprofen increased the participants' production of luteinizing hormone, which usually stimulates the production of testosterone - but this didn't happen during the study. This condition, compensated hypogonadism, usually only appears in the elderly and smokers.
"These results support the possibility that anti-inflammatory drugs like ibuprofen could result in lower testosterone levels if taken over several weeks," said Professor Kelton Tremellen, a gynaecologist and professor of reproductive medicine at Flinders University.
"Sperm quality was not assessed in the study, but... there is some theoretical concern that long-term use of this drug could potentially negatively impact on male fertility potential."
Prof Tremellen said taking it would be "highly unlikely" popping a couple of ibuprofen tablets for a headache would cause any problems, but men should speak to their doctor before making it a daily habit.
Ibuprofen is sold over-the-counter in New Zealand as Nurofen and Advil, and generic versions are sold by most supermarkets.
Whether other non-steroidal anti-inflammatories like aspirin and diclofenac (Voltaren) have similar adverse effects is unclear, but writing for sci-tech news site Ars Technica, biochemist Dr John Timmer suggested it is likely.
"Studies have shown that NSAID exposure during pregnancy was associated with reduced testosterone and congenital malformations; another study showed a drop in a testosterone metabolite among men who were taking ibuprofen regularly."
The findings of the latest research were published in journal Proceedings of the National Academy of Sciences.