Hormone
replacement therapy (HRT) for post-menopausal women was thought to decrease the chance of cardiovascular
problems like heart attack and stroke. This ‘made sense’ because the risk of
cardiovascular disease in women rose sharply after menopause, indicating
that female hormones had a protective effect. Many large observational studies
supported this belief, and HRT was widely prescribed in the 1980s and 1990s.
Later evidence from large, placebo controlled, randomised trials failed to show
any cardiovascular benefit. Again, observational evidence was shown to overestimate the
effectiveness of a common medical treatment and again, practice became established before the definitive trials were done.