Efficacy: Maximal effect a drug can produce. ↑ V_max = ↑ efficacy. Unrelated to potency (ie, efficacious drugs can have high or low potency). Partial agonists have less efficacy than full agonists.

Potency: Amount of drug needed for a given effect. Unrelated to efficacy (ie, potent drugs can have high or low efficacy). ↓ EC₅₀ = ↑ potency = ↓ drug needed.