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The therapeutic index (also known as therapeutic ratio), is a comparison of the amount of a therapeutic agent that causes the therapeutic effect to the amount that causes toxic effects. Quantitatively, it is the ratio given by the dose required to produce the toxic effect divided by the therapeutic dose. A commonly used measure of therapeutic index is the lethal dose of a drug for 50% of the population (LD₅₀) divided by the minimum effective dose for 50% of the population (ED₅₀).

Generally, a drug with a narrow therapeutic range (i.e. with little difference between toxic and therapeutic doses) may have its dosage adjusted according to measurements of the actual blood levels achieved in the person taking it. This may be achieved through therapeutic drug monitoring (TDM) protocols. Examples of drugs with a narrow therapeutic range, that may require drug monitoring both to achieve therapeutic levels and minimise toxicity, include: digoxin, dimercaprol, theophylline, and lithium carbonate. Some antibiotics require monitoring primarily to minimise adverse effects, including: gentamicin, vancomycin, amphotericin B, and polymyxin B.

An important related factor is the leverage of targeting. For example, shaping radiation beams precisely to the profile of a tumor in the "beam's eye view" can increase the delivered dose without increasing toxic effects, though such shaping might not change the therapeutic index. Similarly, when chemotherapy or radiotherapy is infused or injected, the delivery ratio can be much higher when the chemotherapy agent or radioisotope is attached to an oncophilic substance — as is done in peptide receptor radionuclide therapy for neuroendocrine tumors and in chemoembolization or radioactive microspheres therapy for liver tumors and metastases — again, that might not change the therapeutic index.