Communicating the benefits and harms of treatments.

Although various measures can be used to describe the benefits and harms of treatments, not all of these clearly show the benefits or otherwise of treatments in a clinically useful way. Relative risk and relative risk reduction are commonly used to describe the results of studies, but they are of limited clinical usefulness as they do not take baseline risks into account and tend to exaggerate the results of studies. Absolute risk measures such as the number needed to treat (NNT) and the number needed to harm (NNH) allow risk to be expressed in a much more clinically relevant way. The absolute risk measures reflect baseline risk and more accurately indicate the magnitude of the treatment effect. However, because they vary according to the baseline risk of the population, they are of limited generalisability, and the published NNT of a treatment in one population cannot be directly applied to another population with a different baseline risk. There are, however, a number of simple methods which can allow us to estimate NNTs or NNHs for our own patients based on published data. The benefits of a treatment (expressed as the NNT) and the harms of the treatment (expressed as the NNH) can be combined into a single ratio called the likelihood of being helped or harmed (LHH). LHH can be adjusted for individual patients by taking account of their own values and unique circumstances.