An over-the-counter painkiller used by millions can significantly increase the risk of a heart attack or a stroke, research has found. Patients using diclofenac were found to be 40% more at risk than those who were not using the drug, a study published in Plos Medicine found. Diclofenac was the most commonly prescribed non-steroidal anti-inflammatory drug (NSAID) in Britain last year, with six million prescriptions written for it. The drug is also available to buy over the counter. In 2010, almost 17 million prescriptions were filled out for NSAIDs, which are used for pain relief and their anti-inflammatory effects in conditions including arthritis, back pain, gout, headache, and the aches and fever associated with flu. Researchers from the Hull York Medical School and the Institute for Clinical Evaluative Sciences in Canada made the observation about the drug after they analysed 51 international studies into the impact of a range of NSAIDs on more than 2.7 million patients. Lead researcher Dr Patricia McGettigan said: \"NSAIDs provide pain relief for millions of patients with chronic inflammatory disorders. The cardiovascular risk is well described but often overlooked. \"In choosing which one of the many available NSAIDs to use, patients and doctors would benefit from knowledge of the balance between benefit and harm for individual NSAIDs. For example, diclofenac, the NSAID most commonly prescribed in England in 2010, was associated with an increase in cardiovascular risk of 40%, compared with non-use. \"At high doses, the increase in risk was almost doubled. An alternative, naproxen, prescribed only half as often, was not associated with increased risk at any dose. For the first time, we have enough data to make direct comparisons between NSAIDs to determine which are most risky and which are relatively safe.\" Dr McGettigan said that patients should discuss the risk with their doctors, adding: \"In terms of reliving pain, these drugs work and for some people they are the only things that work. It is about weighing up the pain relief against the risk.\" The Medicines and Healthcare products Regulatory Agency (MHRA) warned patients not to stop taking their medicines but to consult their doctor if they are concerned.