partial agonist
A partial agonist is like a dimmer switch that can only turn the lights up to half brightness, no matter how hard you push it. It binds the receptor and switches it on, but even when every receptor is occupied it cannot produce the full response a full agonist would.
The reason is intermediate intrinsic activity (efficacy): a partial agonist stabilizes the receptor's active state less effectively than a full agonist, so its maximal effect (Emax) is submaximal. Affinity and efficacy are separate properties — a partial agonist can bind very tightly yet still cap the response below the ceiling.
A key twist is that, in the presence of a full agonist, a partial agonist acts partly like an antagonist: it competes for the receptor and pulls the overall response down toward its own lower ceiling. This dual behaviour is therapeutically useful — buprenorphine, a partial opioid agonist, relieves pain and curbs withdrawal while carrying a lower overdose risk than full agonists.
Buprenorphine is a partial agonist at the μ-opioid receptor: it eases pain and withdrawal but its ceiling effect on respiration makes fatal overdose less likely than with full agonists like heroin.
A partial agonist activates a receptor but only to a submaximal ceiling.
Whether a ligand looks like a full or partial agonist can depend on the tissue: where there is large receptor reserve, a partial agonist may produce a near-maximal effect.