Drugs to correct heart rhythm abnormalities in patients having a heart attack
Dr Spock’s advice may have seemed logical, but it was based on untested theory. Other examples of the dangers of doing this are not hard to find.
After having a heart attack, some people develop heart rhythm abnormalities – arrhythmias. Those who do are at higher risk of death than those who don’t. Since there are drugs that suppress these arrhythmias, it seemed logical to suppose that these drugs would also reduce the risk of dying after a heart attack.
In fact, the drugs had exactly the opposite effect. The drugs had been tested in clinical trials, but only to see whether they reduced heart rhythm abnormalities. When the accumulated evidence from trials was first reviewed systematically in 1983, there was no evidence that these drugs reduced death rates. 
However, the drugs continued to be used – and continued to kill people – for nearly a decade. At the peak of their use in the late 1980s, one estimate is that they caused tens of thousands of premature deaths every year in the USA alone. They were killing more Americans every year than had been killed in action during the whole of the Vietnam war. 
It later emerged that, for commercial reasons, the results of some trials suggesting that the drugs were lethal had never been reported. [4, See also Cowley et al 1993]