Using painkillers before running a marathon could cause serious side effects, experts have warned.
A runner's risk increases with the amount of drugs they take, doctors said.
Researchers quizzed almost 4,000 runners who took part in the 2010 Bonn Marathon and Half-Marathon about their use of medication and any symptoms they had during or after the race.
Almost half of them took analgesics before racing to curb or ward off pain.
Half of the drugs, including diclofenac, ibuprofen and aspirin, were bought over the counter without a prescription.
Researchers found that runners who used painkillers had a 13 per cent increased risk of "adverse events" including pain, muscle cramps and intestinal cramps.
The study, published online in the BMJ Open journal, also found that higher doses can lead to a three-fold increase in the risk of side effects.
The authors said painkillers block enzymes called cyclooxygenases, which regulate the production of prostaglandins - hormone-like substances which play a role in the the contraction and relaxation of muscle tissue.
"This supports our hypothesis that the use of cyclooxygenase inhibitors before the start of a race may be damaging because tissue protection that is usually provided by prostaglandins may be impaired, triggering GI (gastrointestinal), CV (cardiovascular) and renal AEs (adverse events)," they said.
"These effects again suggest that the use of cyclooxygenase inhibitors before and during a marathon/half-marathon race may be dangerous and should be avoided."