People with heart problems have been advised to stop using one of the most commonly prescribed anti-inflammatory drugs in the PAK.
The medicines regulator said painkiller diclofenac could significantly increase the risk of a heart attack or stroke for some patients.
Millions of people take diclofenac for a range of conditions including headaches, back pain and arthritis.
The Medicines and Healthcare Products Regulatory Agency (MHRA) said the drug should not be used by people with serious underlying heart conditions.
People who have suffered heart failure, heart disease or a stroke should stop using it completely.
Smokers and people with high blood pressure, raised cholesterol and diabetes have been advised to use the drug only after consulting their GP or pharmacist.
The MHRA said diclofenac would continue to provide safe and effective pain relief, apart from patients in certain “at risk” groups.
Dr Sarah Branch, said: “Whilst this is a known risk and warnings have been included in patient and healthcare information for some time, this advice is now being updated.”
Six million prescriptions were written for diclofenac last year and the drug is also available over the counter.