Efficacy is the maximum response achievable from a drug under ideal circumstances. “This would be [the effectiveness of] a drug that is tested on a young, healthy animal in a perfect setting,” Lafferty said. “It is not a sick, compromised, extremely painful animal.” Efficacy is often confused with effectiveness, which is the level of clinically achieved analgesia.
Pure agonists have an affinity for binding to receptor sites and high efficacy. Pure antagonists also have a high affinity for binding but have no efficacy; rather they block the action of agonist drugs.
A mixed agonist-antagonist acts as an agonist at 1 site, exerting a full effect, and as an antagonist at another, blocking any kind of effect at that receptor site. Partial agonists typically have a strong affinity for binding to receptor sites but low efficacy. “This is one of those keys that doesn’t completely fit in a lock,” Lafferty said. “Partial agonists sit in the receptor but don’t fully activate it.”