Two broad families: how the body handles a drug versus what the drug does
Drug interactions split into two categories. Pharmacokinetic interactions change how much drug reaches its target by altering absorption, distribution, metabolism, or transport. For example, one drug can inhibit or induce a metabolizing enzyme, or compete for a transporter, raising or lowering the other drug's blood level. Pharmacodynamic interactions instead change the response at the target without changing concentration, when two drugs act on the same system.
Pharmacodynamic effects can be additive (effects sum), synergistic (combined effect greater than the sum), or antagonistic (one offsets the other). Two sedatives together, or two drugs that both prolong the heart's QT interval, illustrate how additive or synergistic effects can become dangerous even when each drug alone is safe.
Why some interactions matter more than others
An interaction is most clinically significant when the affected drug has a narrow margin between effective and toxic levels, or when the consequence is severe. Many interactions are minor and require no action; others warrant dose adjustment, monitoring, or avoiding the combination. Onset also varies: enzyme inhibition acts quickly, while induction develops over days to weeks.
Following an interaction across the grid
This is general educational information about pharmacology, not medical advice. Interaction risk depends on the specific drugs, doses, and patient, and should be reviewed by a clinician or pharmacist; do not start, stop, or combine medications based on a diagram.