BMJ  2008;336:735 (5 April), doi:10.1136/bmj.39535.475648.1F

Letters

Observational evidence for determining drug safety

Too high a pedestal

The first 150 words of the full text of this article appear below.

Freemantle and Irs are wrong to say that only properly randomised trials can provide truly reliable evidence on adverse events, just as these are the only source of convincing data on drug efficacy.¹ Harms due to drugs differ from benefits in several ways: they are multifarious and affect fewer individuals, some of whom may have particular susceptibilities. Harms often cannot be identified in advance. In some cases these features militate against the practicable use of randomised trials.

If observational studies show no evidence of harms, randomised trials are certainly necessary. They are always desirable, and some adverse effects can be elicited reliably only in this way. However, there are examples of anecdotal reports that provide definitive evidence of both harms and benefits, making randomised trials unnecessary.² There are also examples of adverse effects that have only emerged from observational studies, having failed to be elicited by randomised studies.³

If an . . . [Full text of this article]

¹ Department of Primary Health Care, Oxford OX3 7LF

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