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The number needed to treat (NNT) is an epidemiological
Epidemiology

Epidemiology is the study of factors affecting the health and illness of populations, and serves as the foundation and logic of interventions made in the interest of public health and preventive medicine....
 measure used in assessing the effectiveness of a health-care intervention, typically a treatment with medication
Medication

A pharmaceutical drug, also referred to as medicine or medicament, can be loosely defined as any substance intended for use in the diagnosis, cure, mitigation, treatment, or prevention of disease....
. The NNT is the number of patients who need to be treated in order to prevent one additional bad outcome (i.e. to reduce the expected number of cases of a defined endpoint by one). It is defined as the inverse of the absolute risk reduction
Absolute risk reduction

In epidemiology, the absolute risk reduction is the decrease in risk of a given activity or treatment in relation to a control activity or treatment....
. It was described in 1988.

Variants are sometimes used for more specialized purposes.






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The number needed to treat (NNT) is an epidemiological
Epidemiology

Epidemiology is the study of factors affecting the health and illness of populations, and serves as the foundation and logic of interventions made in the interest of public health and preventive medicine....
 measure used in assessing the effectiveness of a health-care intervention, typically a treatment with medication
Medication

A pharmaceutical drug, also referred to as medicine or medicament, can be loosely defined as any substance intended for use in the diagnosis, cure, mitigation, treatment, or prevention of disease....
. The NNT is the number of patients who need to be treated in order to prevent one additional bad outcome (i.e. to reduce the expected number of cases of a defined endpoint by one). It is defined as the inverse of the absolute risk reduction
Absolute risk reduction

In epidemiology, the absolute risk reduction is the decrease in risk of a given activity or treatment in relation to a control activity or treatment....
. It was described in 1988.

Variants are sometimes used for more specialized purposes. One example is number needed to vaccinate
Number needed to vaccinate

Number needed to vaccinate is a metric used in the evaluation of vaccines, and in the determination of vaccination policy. It is a specific application of the number needed to treat metric that incorporates the implications of herd immunity....
.

Derivation

In general, NNT is computed with respect to two treatments A and B, with A typically a drug and B a placebo
Placebo

The placebo effect is a phenomenon in medicine where the results of a medical treatment are affected by their symbolism, and not just their medical value....
 (e.g., A might be a 5-year treatment with a drug, while B is no treatment). A defined endpoint has to be specified (e.g., the appearance of colon cancer in a five-year period). If the probabilities
Probability

Probability, or wikt:chance, is a way of expressing knowledge or belief that an Event will occur or has occurred. In mathematics the concept has been given an exact meaning in probability theory, that is used extensively in such areas of study as mathematics, statistics, finance, gambling, science, and philosophy to draw conclusions about t...
 pA and pB of this endpoint under treatments A and B, respectively, are known, then the NNT is computed as 1/(pB – pA).

Relevance

The NNT is an important measure in pharmacoeconomics
Pharmacoeconomics

Pharmacoeconomics refers to the scientific discipline that compares the value of one pharmaceutical drug or drug therapy to another. It is a sub-discipline of Health economics....
. If a clinical endpoint is devastating enough (e.g. death
Death

Death is the permanent termination of the biological functions that define a life organism. It refers to both a particular event and to the condition that results thereby....
, heart attack
Myocardial infarction

Myocardial infarction , commonly known as a heart attack, occurs when the Blood flow to part of the heart is interrupted. This is most commonly due to occlusion of a coronary artery following the rupture of a Vulnerable plaque, which is an unstable collection of lipids and white blood cells in the wall of an artery....
), drugs with a high NNT may still be indicated in particular situations. If the endpoint is minor, health insurers
Health insurance

The term health insurance is generally used to describe a form of insurance that pays for medical expenses. It is sometimes used more broadly to include insurance covering Disability insurance or Long term care insurance needs....
 may decline to reimburse drugs with a high NNT.

Example: statins for primary prevention

For example, the ASCOT-LLA manufacturer-sponsored study addressed the benefit of atorvastatin
Atorvastatin

Atorvastatin , is a member of the drug class known as statins, used for lowering blood cholesterol. It also stabilizes plaque and prevents strokes through anti-inflammatory and other mechanisms....
 10 mg (a cholesterol
Cholesterol

Cholesterol is a lipidic, waxy alcohol found in the cell membranes and transported in the blood plasma of all animals. It is an essential component of mammalian cell membranes where it is required to establish proper membrane permeability and membrane fluidity....
-lowering drug) in patients with hypertension
Hypertension

Hypertension, also referred to as high blood pressure, HTN or HPN, is a medical condition in which the blood pressure is chronically elevated....
 (high blood pressure) but no previous cardiovascular disease (primary prevention). The trial ran for 3.3 years, and during this period the relative risk
Relative risk

In statistics and mathematical epidemiology, relative risk is the risk of an event relative to exposure. Relative risk is a ratio of the probability of the event occurring in the exposed group versus a non-exposed group....
 of a "primary event" (heart attack) was reduced by 36% (relative risk reduction, RRR). The absolute risk reduction (ARR), however, was much smaller, because the study group did not have a very high rate of cardiovascular events over the study period: 2.67% in the control group, compared to 1.65% in the treatment group. Taking atorvastatin for 3.3 years, therefore, would lead to an ARR of only 1.02% (2.67% minus 1.65%). The number needed to treat to prevent one cardiovascular event would then be 99.7 for 3.3 years.

Worked example


The relative risk (odds ratio) is .25 in the example above. It is always 1-relative risk reduction, or vice versa.

See also

  • Number needed to harm
    Number needed to harm

    The number needed to harm is an epidemiology measure that indicates how many patients need to be exposed to a risk-factor to cause harm in one patient that would not otherwise have been harmed....
     — the converse for side-effect
    Adverse effect (medicine)

    In medicine, an adverse effect is a harmful and undesired effect resulting from a medication or other intervention such as chemotherapy or surgery....
    s


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