What Is a Narrow Therapeutic Index?

A narrow therapeutic index is a very small range of doses at which a medication provides benefits without causing severe and potentially fatal complications. This means that very small dosing changes could have a severe impact on outcomes for the patient. Care providers use caution when working with drugs that have a narrow therapeutic index to reduce risks for patients. It is important to follow directions given with such medications carefully, as a small mistake could be fatal.

The therapeutic index is the range of doses at which a medication appeared to be effective in clinical trials for a median of participants. It is expressed as the range between the median effective dose, known as ED50, and the median toxic dose, TD50. Patients who take medications at low doses may not experience benefits, while patients who take medications at higher doses could be at risk of severe complications or death. The range between the ED50 and TD50 can be considerable, depending on the medication.

If the difference between the two extremes is twofold or less, a medication has a narrow therapeutic index. Patients who accidentally take two tablets instead of one, for example, could be in the lethal dose range. This doesn’t mean that they will necessarily die, because the TD50 is an expression of a median, not every single member of a study. Additionally, because killing people in clinical trials to find out how much medication is fatal is not ethically acceptable, pharmaceutical companies calculate the therapeutic index using factors like the development of severe complications in trial participants. The TD50 is based on animal studies and the evaluation of human clinical trials to develop a reasonable estimate, erring on the side of caution.

Patients on medications with a narrow therapeutic index must be careful because overdosing could endanger them. They need to make sure to follow a dosing schedule and to read directions carefully. This can include examining tablets or suspensions before using them to check for signs of abnormalities, like markings that do not look familiar. Some examples of medications with a narrow therapeutic index include lithium, digoxin, and warfarin.

Care providers also need to be wary of such medications and facilities like clinics and hospitals may have special rules for their administration. This can include requiring doctors to fully write out prescriptions to eliminate confusion about dosage, asking nurses to double check each other before administering narrow therapeutic index drugs, and using special labeling on such medications. For example, a nurse preparing a saline drip infused with a dangerous drug might use a special color-coded label to warn other care providers about the contents.