Thousands of people who take prescription medicines for everyday conditions are gaining large amounts of weight as an unexpected side effect, scientists have warned.

Researchers, who found that some patients were putting on up to 22lbs in a year, say that the drugs may even be contributing to the nation's rocketing obesity epidemic.

All of the patients they studied, on medication for conditions as diverse as diabetes, epilepsy, depression, high blood pressure and schizophrenia, showed evidence of weight increase.

"Given the common and long-term use of many of these drugs," said the researchers, "it is likely that they play a significant contributory role in the increasing prevalence of obesity."

A team from Glasgow University and Glasgow Royal Infirmary reviewed and analysed data on drug use by more than 25,000 people to quantify the effects of prescription drugs.

All of the drugs included in the review are used to treat chronic diseases by large numbers of people. In the UK, it is estimated that around 2.6 million people have been diagnosed with coronary heart disease, for which beta-blockers are widely prescribed. More than 400,000 people have epilepsy, and around 1.3 million people have type 1 or type 2 diabetes.

"In the light of these figures, the number of individuals in the population receiving treatment with an obesogenic drug is potentially quite high. In Scotland alone, the number of prescriptions dispensed for beta-blockers and tricyclic antidepressants between 2004 and 2005 exceeded one million and two million respectively," says the report.

With many of the drugs, weight gain was significant and rapid, according to the researchers. Just how they lead to weight gain varies, and is in some cases unknown. Some, like corticosteroids, increase appetite, while beta-adrenoceptor blockers reduce metabolic rate.

Olanzapine and clozapine, drugs used for psychiatric conditions, resulted in the most weight gain - up to 22lbs in 52 weeks. Insulin for type 2 diabetes was found to increase weight by up to 13.2lbs, while some drugs for depression added up to 8lbs, and some for high blood pressure led to gains of up to 3lbs. A heart drug added 5lbs, while some treatments for epilepsy added more than 12lbs, and some bipolar drugs led to weight gains of around 8lbs.

The researchers say that many other drugs which are being prescribed and have not been investigated may also have an effect on weight.

They also suggest that unexpected weight gain may be a reason why some people stop taking prescribed drugs: "Non-compliance with any drug therapy is a widespread problem, and around half of patients prescribed long-term medication for the management of chronic diseases do not comply fully with treatment.

"Non-compliance is reported as an issue with many of the drugs included in this review, and the weight gain associated with them may contribute to this."

In future, the scientists suggest, doctors should discuss with patients the risk of weight gain before they start treatment: "This review provides evidence of the weight gain potential of some common drugs. It is perhaps only now, in light of the present epidemic of obesity, that the negative effect on body weight is a pertinent issue. The potential of weight gain should be discussed with patients prior to the institution of therapy."

effects: Prescription problems

The drugs listed below are prescription medicines implicated in the new research. Each has been identified as possibly being responsible for dramatic weight gain:


Used to treat high blood pressure, abnormal heart rhythms and panic attacks. The drugs block the effects of adrenalin, slowing the heart rate.


Generally known as steroids, the drug is used to treat various conditions, from rheumatoid arthritis to stress relief after a trauma or operation.


Used to treat schizophrenia and manic episodes. Is said to help the patient stay in touch with reality.