A drug company suppressed research which showed that generic thyroid drugs were as effective as its own branded product for almost seven years, says the Journal of the American Medical Association.

A randomised four way crossover trial concluded that two brand name and two generic forms of thyroxine sodium (levothyroxine) were bioequivalent and interchangeable without loss of therapeutic efficacy in most patients for the treatment of hypothyroidism (JAMA 1997;277:1205-18). The two brand name products were Synthroid–the most commonly used brand in the United States–and Levoxine (now renamed Levoxyl)–a newer, cheaper product similar in price to generic forms.

Thyroxine sodium is widely prescribed in the United States as the drug of choice for thyroid replacement treatment. The Food and Drug Administration (FDA) does not recognise thyroxine sodium products as bioequivalent, and generic substitution is not recommended. The authors of the study estimate that using generic or less expensive brand name products in the United States could save $356m (£223m) a year.