Three words that are not the same
People use "side effect", "adverse reaction", and "toxicity" as if they meant one thing. In pharmacology they don't. A side effect is any effect of a drug other than the one you wanted — it can even be helpful. An adverse drug reaction (ADR) is a response that is harmful and unintended, occurring at normal doses. Toxicity is harm that comes from too much drug — too high a dose, or the right dose building up in someone who cannot clear it.
A useful test: aspirin thins the blood. In someone with a heart attack that is the wanted effect. In someone about to have surgery it is an unwanted side effect — and in someone bleeding from an ulcer it is a harmful ADR. Same drug, same action; only the context changed.
The A/B framework: predictable vs strange
Most ADRs fall into two big buckets. Type A ("Augmented") reactions are an extension of the drug's own pharmacology — too much of what it normally does. They are dose-related, predictable, common, and usually reversible by lowering the dose. Bleeding on a blood thinner, low blood sugar on insulin, drowsiness on a sedative: all Type A.
Type B ("Bizarre") reactions are not predictable from the known action of the drug. They do not depend on dose, are rarer, and can be severe — drug allergies and idiosyncratic reactions live here. A tiny dose can trigger a life-threatening response in a susceptible person. The split matters because Type A is managed with the dose dial, while Type B usually means stopping the drug and never giving it again.
TYPE A (Augmented) TYPE B (Bizarre) - extension of drug action - not predictable from action - dose-dependent - dose-independent - common, low fatality - rare, higher fatality - manage: lower the dose - manage: stop, avoid forever e.g. warfarin -> bleeding e.g. penicillin -> anaphylaxis
Why harm is the price of action
Drugs cause harm for two simple reasons. First, no drug is perfectly selective: the same molecule that hits the intended target also nudges related targets in other tissues. An antihistamine meant for your nose also blocks receptors in your brain, so you feel sleepy. Second, even a perfectly selective drug does the right thing too strongly when there is too much of it — that is dose-dependent toxicity.