How many pills are too many?

Even more useful in understanding a drug’s therapeutic impact are figures known as Number Needed to Treat (NNT) and Number Needed to Harm (NNH). NNT is the inverse of the absolute risk reduction and indicates the number of patients who need to receive a treatment for one patient to achieve the desired benefit. For instance, an NNT of five means if five patients receive a treatment then one patient benefits.

Number Needed to Harm is the number requiring treatment before one patient is harmed. In many clinical situations, it is pretty much unknown. Try asking your doctor for the NNT and NNH of treatment you receive. They probably won’t know. I usually don’t.

Low NNTs are good whereas low NNHs are bad. Highly effective treatments, for instance antibiotics for susceptible infections, have excellent NNTs under two. Preventive measures, for instance drugs to reduce high blood pressure or high cholesterol, often have disappointing NNTs of over 100 in all but the very highest risk groups.

With so many patients on so many drugs, it is worth considering whether every drug they take is actually essential. For instance, a patient on ten drugs with respectable NNTs of five would appear to have about one chance in ten million that every drug being taken is of benefit and over one chance in ten that absolutely none of the ten drugs taken are helpful.

Incidentally, the NNT for regular exercise to prevent a heart attack is perhaps about three. In other words, compared with high-tech drugs, exercise is vastly more effective at preventing heart attacks and is almost certainly less harmful. No wonder an apocryphal tsar had an interesting solution when he discovered his most disease-ridden province also had the most doctors. He killed the doctors.