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It takes about five half‐lives to get to the steady‐state concentration after administering multiple dosages of a drug.

Drug	Dosage interval, τ	Halt‐life, t½	Time to steady state,
Lignocaine	1 h	90 min	7.5 h
Theophylline	12 h (MR*)	8 h	40 h
Digoxin	24 h	48 h	10 days
Amiodarone	24 h	50 days	250 days

* modified release dosage form.

Therapeutic drug monitoring (TDM) involves frequent checking of the plasma drug concentration in the patient and consequent adjustment of the dose to ensure plasma concentrations are maintained within the desired therapeutic range. TDM is usually performed for drugs that are known to exhibit significant pharmacokinetic variability, those with a very narrow therapeutic range such as potent drugs or those where the pharmacological response strongly depends on maintaining a target drug concentration in the body (Box 2.7). Often such variability in pharmacokinetics is due to intra‐ and inter‐subject variability in drug absorption, metabolism or elimination processes. The differences in rate and extend of drug absorption (often due to physiological or formulations related factors) may have implications in steady‐state pharmacokinetics and can therefore affect therapeutic efficacy.