At least one out of seven commonly used prescription medications are used for off-label purposes. These other uses often lack scientific support, a new study says.

Researchers used data from the 2001 IMS Health National Disease and Therapeutic Index examination of 160 common drugs. They found that about 151 million prescriptions — 21% of 725 million prescriptions — were for off-label use. Furthermore, three-quarters of the off-label uses lacked scientific support, said researchers led by Randall Stafford, assistant professor of medicine at Stanford Prevention Research Center.

Off-label prescribing was most common with heart drugs (excluding those for cholesterol and high blood pressure), anticonvulsants and asthma medications. Medications for diabetes blood sugar control, pain relief and high cholesterol were the least likely to be prescribed for off-label purposes. The study report was published Tuesday in the Archives of Internal Medicine.