Most drugs exert their effects on the body through activities at receptors. Receptors are proteins that are usually located on the surface of cells. After binding these receptors, drugs can produce a response from the cell, or prevent other compounds from using that receptor to do so. The antagonist drug class falls into the latter category, effectively ensuring that the cell does not respond to activating compounds, known as agonists, in their presence. These agonists can include other drugs, or the brain's natural communication chemicals, called neurotransmitters.
Depending on the nature of the antagonist drug, they may create their effects in different ways. Competitive antagonists bind to the same location on the receptor as agonists, and prevent these agonists from binding. Another type, non-competitive antagonists, will bind other parts of the receptor. Once bound, they may diminish the ability of the receptor to become activated, or may even prevent them from activating at all while the antagonist is present. Most antagonists are reversible, and eventually leave the body, but a small number of these substances are irreversible, causing permanent effects after they are taken.
There are many medical uses for drug antagonists. One antagonist drug group, beta blockers, preferentially binds to receptor sites called beta adrenergic receptors. Usually, these receptors allow the neurotransmitter epinephrine to bind them and cause the cells to pass along electrochemical messages in an act known as firing. Overactivity of epinephrine at these receptors is sometimes a part of hypertension and other medical conditions. Beta blockers, by acting as an antagonist, keep epinephrine from exerting its effects, thereby alleviate the symptoms of hypertension.
Some mental illnesses can respond to treatment with an antagonist drug. Schizophrenia, for example, seems to be linked to abnormal activities of the neurotransmitter dopamine. Antipsychotic drugs sometimes function as dopamine antagonists, and can relieve some of the hallucinations and altered thought patterns caused by this disorder. Not all schizophrenia symptoms can be treated with an antagonist drug, however.
Another function of receptor antagonists is to treat drug overdoses. Opioid painkillers can cause coma or death in large dosages, but opioid antagonists such as naloxone can reverse this effect. Naloxone competes with the opioids for receptor space, and prevents these drugs from working properly after it is administered. Benzodiazepine overdose may sometimes be treated in a similar fashion with a different drug that competitively binds the same receptor, known as flumazenil.