Taking common painkillers while on the pill may lead to a small increased risk of blood clots, research suggests.
Experts said that while the risk is low, women should be informed of the link between non-steroidal anti-inflammatory drugs (NSAIDs) and hormonal contraception.
NSAIDs are widely used to relieve pain, fever and to reduce inflammation.
The main types of NSAIDs in the UK include ibuprofen, naproxen, diclofenac, celecoxib, mefenamic acid, etoricoxib and indomethacin.
The Danish study, published in the British Medical Journal (BMJ), included data for more than two million women.
It found that the risk was greater in women using ibuprofen, diclofenac and naproxen while on combined pills containing progesterone and oestrogen, but was smaller in women using progestogen-only pills (often called the mini-pill), implants and coils.
The researchers 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.
Hormonal contraception was divided into high, medium and low risk, according to their links with clots in previous studies.
The researchers said high risk, hormonal contraception included combined pills containing either 50mcg oestrogen or third or fourth generation progestogen, while medium risk included all other combined oral contraceptives.
Low or no risk included the mini pill, implants and coils.
The study found that NSAIDs were used by 529,704 women on hormonal contraception, with ibuprofen being the most frequently used NSAID (60%), followed by diclofenac (20%) and naproxen (6%).
Over an average of 10 years, 8,710 venous clots occurred (2,715 pulmonary embolisms and 5,995 deep venous thromboses), and 228 (2.6%) women died within 30 days of diagnosis.
The study found NSAID use was associated with four extra venous thromboembolic events per week per 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.
They said: “Women needing both hormonal contraception and regular use of NSAIDs should be advised accordingly.”
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, Imperial College London, said: “Blood clots are dangerous since they can lodge in the lungs causing breathing and heart problems.
“Contraceptive medications and painkillers like ibuprofen are essential for many women to avoid pregnancy and cope with period pain.
“Painkillers and the pill (like all drugs) can occasionally cause serious side-effects.
“But I don’t think this study alone should put off women taking either the pill, painkillers, or both if needed.
“The most important message should be for all women to reduce blood clot risk by smoking cessation and weight loss.”