Drug–Receptor Interactions

Agonists, Partial Agonists and Antagonists: The Efficacy Spectrum

Two drugs can both grip the same receptor and yet do opposite things — one switches the cell full on, the other jams the lock so nothing else can. Between them sits a stranger drug that turns the cell only halfway on and, in the wrong company, actually turns it down. Binding is only half the story; what a drug DOES once bound — its efficacy — is what separates a painkiller from an antidote. This is the spectrum that makes pharmacology click.

Affinity, Efficacy and Potency: Three Words Students Always Confuse

A patient asks for the "strongest" painkiller. Behind that one word hide three different ideas that decide everything a drug does: whether it even binds its receptor, what it does once bound, and how many milligrams you need. Mix them up and you will believe a microgram of fentanyl is somehow "better" than a milligram of morphine, or that the 10 mg tablet beats the 40 mg one. Untangle affinity, efficacy and potency and you can read any dose–response curve — and any drug advert — without being fooled.

Types of Antagonism: Competitive, Irreversible, Chemical and Physiological

An antagonist is any drug that opposes another — but there is more than one way to say no. One competes for the same seat and can be shouted down by more agonist; another locks the door for good and no amount of agonist will reopen it; a third never touches the receptor at all, neutralising its rival by chemistry, by clearance, or by pushing the opposite lever through a different pathway. Sort these four families and you can predict every dose–response curve on the exam — and rescue a patient in anaphylaxis.