Consider aspirin for heart attack prevention. Based upon both modifiable risk factors like cholesterol level and smoking, and factors that are beyond one’s control, like family history and age, it is possible to calculate the chance that a person will have a first heart attack in the next 10 years. The American Heart Association recommends that people who have more than a 10 percent chance take a daily aspirin to avoid that heart attack.
How effective is aspirin for that aim? According to clinical trials, if about 2,000 people follow these guidelines over a two-year period, one additional first heart attack will be prevented.
That doesn’t mean the 1,999 other people have heart attacks. The fact is, on average about 3.6 of them would have a first heart attack regardless of whether they took the aspirin. Even more important, 1,995.4 people would never have a heart attack whether or not they took aspirin. Only one person is actually affected by aspirin. If he takes it, the number of people who remain heart attack-free rises to 1996.4. If he doesn’t, the number remains 1995.4. But for 1,999 of the 2,000 people, aspirin doesn’t make any difference at all..
There is a complementary metric known as the number needed to harm, or N.N.H., which says that if that number of people are treated, one additional person will have a specific negative outcome. For some treatments, N.N.T. can be higher than the number needed to harm, indicating more people are harmed than successfully treated.)..
For those at higher risk, who have already had a heart attack, to avoid one additional death, the N.N.T.(for Meditteranean diet) is about 30. That’s the number of people who would have to adhere to the diet for four years so that one extra person survived. About 1.4 people out of 30 such people will die no matter what they eat; 27.6 will not die no matter what they eat. Only one will benefit from sticking to the diet...
Steroids work very well for asthma attacks — better than many treatments for other conditions. But still, seven of eight people suffering an asthma attack see no benefit at all from steroids with respect to preventing hospitalization...
Antibiotics are a classic example of overuse. For instance, the N.N.T. for antibiotics to treat radiologically diagnosed acute sinusitis is 15, meaning that 14 out of 15 who take them derive no benefit. But physicians often write prescriptions for antibiotics in situations when the diagnosis of sinusitis is far less assured. This leads to antibiotics being overprescribed and overused, raising their N.N.T. in practice.