Women are being warned that taking some common painkillers while using the contraceptive pill may increase their risk of suffering blood clots.
A Danish study of two million women found using ibuprofen, diclofenac and naproxen while also on combined pills containing progesterone and oestrogen could lead to a small rise in the chances of having a blood clot.
But women using progestogen-only pills (often called the mini-pill), implants and coils faced a lower risk.
Despite the low danger level, the researchers called for women to be told of the link between non-steroidal anti-inflammatory drugs (NSAIDs) and hormonal contraception.
Three of those with the highest risk-factor – Ibuprofen, naproxen and diclofenac – are among the main types of NSAIDs used in the UK, along with celecoxib, mefenamic acid, etoricoxib and indomethacin.
The team used national medical records to track diagnoses of venous thromboembolism (blood clots) among two million women aged 15 to 49 living in Denmark between 1996 and 2017 with no history of blood clots, cancer, hysterectomy or fertility treatment.
More than half a million (529,704) women taking hormonal contraception used NSAIDs, the study found, with ibuprofen (60%) the most frequently used, followed by diclofenac (20%) and naproxen (6%).
A little less than 9,000 (8,710) venous clots occurred over an average of 10 years, and 228 (2.6%) of the women died within 30 days of being diagnosed.
NSAID use was linked to four extra venous thromboembolic events per week for every 100,000 women not using hormonal contraception, 11 extra events in women using medium risk hormonal contraception, and 23 extra in women using high risk hormonal contraception.
The link was strongest for diclofenac compared with ibuprofen and naproxen, the paper found.
The experts warned the study did not prove a link but said women should be given information about the risks.
A range of factors that could influence the results, such as age, education level, pregnancy history, prior surgery, high blood pressure and diabetes, were taken into account when analysing the findings.
Dr Channa Jayasena, clinical senior lecturer and consultant in reproductive endocrinology and andrology at Imperial College London, said: “Contraceptive medications and painkillers like ibuprofen are essential for many women to avoid pregnancy and cope with period pain.
“But I don’t think this study alone should put off women taking either the pill, painkillers, or both if needed.”