A survey of major drug classes

Bacteria are complete living cells. They have a cell wall, their own ribosomes for making proteins, and enzymes for building the molecules they need. Many antibiotics exploit exactly these features: a penicillin interferes with the building of the cell wall, while other antibiotics bind the bacterial ribosome and stop protein manufacture. Because human cells have no cell wall and use different ribosomes, the drug can harm the bacterium at concentrations the patient tolerates — the principle of selective toxicity. A virus is not a cell at all. It is a small package of genetic material that cannot reproduce on its own; it must enter a human cell and use that cell’s machinery to copy itself. There is therefore no bacterial-style target to attack. Antiviral drugs must instead block a step specific to the virus: its entry into the cell, its own enzymes for copying its genetic material, or its release from the cell. This is why antivirals are harder to develop, why they often work best when started early, and why the antiretrovirals used against HIV are usually combined so that the virus cannot easily escape a single drug. It is also why an antibiotic given for a viral illness is not merely useless but harmful: it selects resistant bacteria in the patient’s body and adds the risk of allergy, stomach upset and other side effects.

When you learn that a drug belongs to a class, you acquire two things at once. The first is a promise: drugs in a class usually share the same therapeutic purpose, so the name tells you roughly what the medicine is for. The second is a warning: they often share the same unwanted effects and the same interactions, because they act by the same mechanism. NSAIDs relieve pain and inflammation, and as a family they irritate the stomach lining; this is why the notes single them out when describing the disadvantages of the oral route. Opioid analgesics relieve severe pain, and as a family they depress breathing and can cause dependence. Corticosteroids suppress inflammation powerfully, and as a family they affect many body systems when used systemically. Antibiotics treat bacterial infection, and as a family their unnecessary use drives resistance. A prescriber who thinks in families can therefore anticipate problems before they appear, warn the patient, and avoid adding a second drug with the same family risk. The limit of the method is that classes are human inventions: two drugs in one class may differ in how long they act, how they are cleared, or how safe they are in pregnancy, and a drug may belong to more than one class depending on dose and purpose, as low-dose aspirin shows. Treat the class as a first map and the formulary as the detailed survey.