Which term refers to the maximal response obtained by a drug?

The term that refers to the maximal response obtained by a drug is efficacy. Efficacy is a crucial concept in pharmacology, as it indicates the drug's ability to produce a desired therapeutic effect when it binds to a receptor or target site in the body. A drug with high efficacy can elicit a strong response, making it very effective in treating a specific condition or disease.

In contrast, potency relates to the amount of drug needed to produce a certain effect; a potent drug achieves the desired effect at lower doses. The therapeutic range is the concentration range in which a drug is effective without being toxic, and the action level is not a standard term used in pharmacology to describe a drug's response. Hence, efficacy is the most appropriate term to indicate the maximal response achievable by a drug.