Lock and Key

Most drugs work by binding to receptors — specific proteins on or inside cells that trigger biological responses. A receptor is like a lock, and the drug is a key.

Morphine is an agonist — it fits opioid receptors in the brain and switches them on, mimicking your body's natural painkillers (endorphins). Naloxone is an antagonist — it fits the same receptors but blocks them, so it can reverse an overdose.