Number needed to treat (NNT) is a statistic in epidemiology referring to how many people need to receive a treatment to prevent a single poor outcome. People use this statistic in weighing the costs and benefits of a treatment and in writing policy recommendations. If the number needed to treat is one, where medical providers only need to treat one person to prevent a bad outcome, this argues strongly in favor of providing the treatment. If 1,000 people need treatment to prevent a bad outcome, the treatment may not be effective enough to include in blanket recommendations.
To determine the number needed to treat, people need two sample populations with the same medical condition. One population receives treatment, like insulin therapy for diabetes, while the other is a control population and receives no medical interventions. People compare the success rates between the populations. In a case where no one in the control group improves and people receiving the treatment get better, the number needed to treat is one.
This can have important implications for doctors making decisions about how to proceed with medical care. In a situation where the doctor knows treatment will be successful, the doctor knows it will be worth the associated risks. If the number needed to treat is higher, the situation becomes more complicated, and the doctor needs to think about the effectiveness of the treatment, the risks, and the patient's case.
This also has important policy implications. Insurance providers, for example, can deny coverage on the basis that the number needed to treat is so high that the efficacy of a treatment is questionable. They won't pay for treatments that only appear to be successful in limited numbers of patients. People may be able to appeal, providing supportive evidence to show how the treatment will benefit them. Policymakers can also consider this when reviewing applications for new drugs and treatments; if a treatment does not seem very effective, it may not be in the interest of the public to approve a request to sell it.
People may also consider the number needed to harm. This refers to how many people receive treatment before people experience a bad outcome because of the treatment. While many medical treatments carry risks, some are riskier than others. If this number is low, a treatment may not be acceptable for use except in very limited circumstances. An invasive surgery with serious risks of complications, for example, should only be undertaken when absolutely necessary, on a patient who has the capacity for informed consent.