Some drugs are forgiving. Take a little too much and nothing bad happens. Others are not. The therapeutic index is a ratio that compares the dose that causes toxicity to the dose that produces the desired effect. A high therapeutic index means there is a wide margin between effective and dangerous doses. A low therapeutic index means the margin is narrow, and small changes in dose or blood level can tip a patient into toxicity.

Warfarin, digoxin, lithium, and phenytoin are classic examples of drugs with narrow therapeutic indices. They require regular blood monitoring to keep levels in the therapeutic range. Even a small interaction with another drug or a change in diet can push them out of range. Drugs with wide therapeutic indices, like penicillin, are easier to use because the effective dose is far below the toxic dose. The therapeutic index is calculated from animal studies, but it is not a perfect predictor of human safety. Some drugs have a wide index in animals but cause rare idiosyncratic reactions in humans.

What the therapeutic index affects:

• Dosing. Narrow-index drugs require careful titration.
• Monitoring. Blood tests are often needed to check levels.
• Interactions. Small changes can have large effects.
• Formulation. Small differences in bioavailability can matter.

For narrow-index drugs, generic substitution may require extra caution, though regulators require bioequivalence within strict limits.