What is a drug interaction, and which ones actually matter?
One substance changing how another behaves in the body — either how much of it reaches the bloodstream and for how long, or what effect it has once there. Most interactions are theoretical or minor; a small number are genuinely dangerous, and they are not always the ones people worry about.
The two categories:
Pharmacokinetic — affecting absorption, distribution, metabolism or excretion. The most important route is the liver enzyme system, where one drug can inhibit or induce the enzymes that clear another. Inhibition raises the other drug's level, sometimes into toxicity; induction lowers it, sometimes into failure.
Pharmacodynamic — two drugs acting on the same system, adding their effects together. Several sedating drugs together, or several that increase bleeding risk.
The interactions that genuinely matter most:
Anticoagulants with many common medicines and supplements, where the consequence is bleeding or clotting.
Drugs with a narrow therapeutic range — where the effective and toxic doses are close — including lithium, digoxin and some epilepsy medicines.
Multiple sedatives together, including opioids with benzodiazepines and alcohol, which is a leading cause of serious harm.
Several drugs raising serotonin, which can produce serotonin syndrome.
NSAIDs with blood pressure and kidney medicines, a very common combination in older people.
The non-drug interactions people forget:
Grapefruit juice, which inhibits an intestinal enzyme and can substantially raise levels of certain drugs — the effect lasts far longer than the juice does, so separating them by hours does not help.
St John's wort, a herbal product that induces enzymes powerfully and reduces the effectiveness of many medicines, including hormonal contraception and transplant drugs.
Alcohol, with a wide range of medicines.
Supplements, which are not inert — iron, calcium and antacids reduce absorption of several antibiotics and thyroid medicine.
What actually protects you: using one pharmacy, keeping a current list of everything including supplements, telling prescribers about anything bought over the counter, and asking the pharmacist, who is the most accessible expert on exactly this.
General information, not medical advice.