Eli Lilly and Amylin Pharmaceuticals have been forced to ask doctors to limit prescriptions for its diabetes drug Byetta after manufacturing capacity for the drug failed to meet a recent increase in demand.

The increase in demand is thought to be a result of data showing that Byetta (exenatide) can prevent diabetes patients from gaining weight, a major problem with existing drugs to treat the disease.

At the moment the cartridges used to deliver Byetta are supplied by UK manufacturer CP Pharmaceuticals, and Lilly has contracted US company Baxter Pharmaceuticals to increase its manufacturing capacity. But this new supply is not expected to come online until later this year.

Lilly has asked doctors to give scale down prescriptions for new patients to make sure that those already on the drug do not suffer an interruption in supply. The company stopped providing samples and vouchers for the drug in April.

The company was expecting a surge of prescriptions on the back of new data, presented at the American Diabetes Association annual conference earlier this week, which showed that Byetta can control blood glucose in patients who are not meeting treatment goals on thiazolidinedione drugs, with or without metformin.

Exenatide was launched onto the US market a year ago under and achieved sales of $67 million in the second half of the year. Amylin and Lilly have also filed for approval of the product in Europe.

Meanwhile, Novo Nordisk has presented Phase III data on a Byetta rival, liraglutide, at the ADA. More details to follow.