Non-clinical sciences/Pharmacology principles · Prescribing safety

Therapeutic drug monitoring

What it means

Drugs with a narrow therapeutic index need plasma-level monitoring: vancomycin, gentamicin/aminoglycosides, digoxin, lithium, phenytoin, theophylline, ciclosporin/tacrolimus, methotrexate. Timing matters (e.g. lithium 12 h post-dose; gentamicin trough).

Worked example

Lithium has such a narrow window that levels, renal and thyroid function are monitored regularly; toxicity → tremor, ataxia, confusion.

In the exam

A mood-stabiliser with a very narrow therapeutic window requires regular plasma-level checks plus renal and thyroid monitoring.

What settles it

TDM is for narrow-therapeutic-index drugs; most drugs are dosed clinically without levels.

Classically confused with

Teratogenic drugs

Source: BNF