It is common for doctors to provide special stockings to stroke patients to prevent blood cuts. But a new study finds that they not only fail to reduce the chances of a clot, but they cause problems such as skin ulcers and blisters.

The study examined more than 2,500 stroke patients in Australia, Britain and Italy. Results were published in the Lancet medical journal and presented at the European Stroke Conference in Stockholm on Wednesday.

In the study, about half of the patients got standard care as well as the tight, thigh-high stockings. The other half just received standard care. Experts took an ultrasound of patients’ legs after about 7 to 10 days, and then again after 25 to 30 days. About 10% of patients in both groups developed blood clots. In the group wearing stockings, 5% reported side effects like skin problems and blisters. Only 1% in the group not given the stockings reported these side effects.