We need to fully understand the benefits, side-effects and risks of a drug before embarking on treatment. Prescription drug information from advertising or brochures employs the clever use of medical concepts that require interpretation. Advertisers rely on our ignorance of such matters and drugs are often made to seem more effective than they really are. Many of us might choose not to take a particular medication if we understood the very small absolute benefit on offer.
The most common way of presenting the benefit of a drug is to use the observed percentage risk reduction for people taking it. So it is common to hear statements like ‘Aspirin offers men a 32% reduction in risk of heart attack’, or ‘Fosamax offers a 50% reduction in risk for hip fracture’. But these impressive sounding percentages only provide the relative risk reduction and not the more telling absolute risk reduction.
Fosamax ads are a good example. The majority of people don’t fracture their bones. Even ‘high risk’ women and men are at low absolute risk for fracture. In the large trial that measured the fracture benefit of Fosamax, it was found that in a population of 2,027 postmenopausal women with osteoporosis and previous fracture (a high risk group) over a three year period, there were 22 hip fractures in the 1000 women in the placebo group, and 11 hip fractures in the 1000 women who were taking the drug, (a difference of just 11 fractures). Thus 2.2 percent of the placebo group fractured, and 1.1 percent of the Fosamax group fractured. Because 1.1 percent is 50% of 2.2 percent, Fosamax is claimed to reduce hip fractures by 50%. This is a relative risk reduction. The actual or absolute reduction is only 1 percent!
Another and sometimes better way to look at the effectiveness of a drug is to consider the numbers needed to treat or NNT. Numbers needed to treat tend to give a picture of a drug’s effectiveness in terms of the number of people who need to take it in order for one of them to benefit. Aspirin for example, has an NNT of 3.2 when used for migraine headache. That means, approximately three people with migraine symptoms would need to use it in order for one of them to get relief. This is considered an effective treatment. With Fosamax, 90 high risk women would need to be treated for three years in order to prevent one hip fracture in one of them. [i] The remaining 89 would receive no benefit and be exposed to the risks and side-effects of the drug. It is estimated that hundreds of women aged 50 years with low bone density would need to be treated for more than 3 years to prevent one hip fracture in one of them.[ii]
It is a good idea to ask your doctor what the NNT is for a particular drug and what the absolute risk reduction is.
[i] Black DM. Randomised trial of effect of alendronate on risk of fracture in women with existing vertebral fractures. Fracture Intervention Trial Research Group. Lancet 1996;348(9041):1535-41.
[ii] Cummings SR. Effect of alendronate on risk of fracture in women with low bone density but without vertebral fractures: results from the Fracture Intervention Trial. JAMA1998;280(24):2077-82.
