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Medicines are often not taken as just a single dose and repeat administration of dosages is often required to achieve remission. In chronic conditions, such as diabetes or hypertension, medicines are administered regularly to maintain the plasma concentration of drugs constantly within the therapeutic window to ensure maintenance of the therapeutic response. After multiple dosage administrations, when a pharmacokinetic equilibrium is achieved, the plasma concentration becomes constant (fluctuates in a constant pattern within the therapeutic window), termed as the ‘steady state’.

At the steady state, the rate of drug input into the systemic circulation becomes equal to the rate of drug output (elimination); hence, a pharmacokinetic equilibrium keeps a fairly constant level of drug concentration in the body. It takes about three to five half‐lives for a drug to get to the steady state (Figure 2.11). This may mean that a drug with t_½ = 8 h if taken twice a day will achieve a steady‐state plasma concentration in 40 hours (Box 2.6). For some drugs, this steady‐state plasma concentration needs very close monitoring during treatment for a patient, termed as therapeutic drug monitoring (TDM).

Figure 2.11 Repeat dose pharmacokinetic profile showing steady state compared to the first dose.