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Number of t½ elapsed	Percentage of drug eliminated	Drug	Half‐life (h)	Time for 90% elimination
1	50	Gentamicin	2–3	7–10 h
2	75
3	87.5	Theophylline	6–8	24 h
3.3	90
4	94
4.3	95
5	97	Lithium	15–30	2–4 days
6	98.4
6.6	99
7	99.2	Digoxin	36–51	5–7 days

Typically, it takes three to five half‐lives for most of the drug to be eliminated from the body.

For a drug with elimination rate constant, k = 0.1 h⁻¹, the half‐life will be ~7 hours. If 100 mg of the drug was present in the body, it will take about seven hours to reduce this to 50 mg by elimination, i.e., to the half; this is also represented by the amount eliminated every hour using elimination rate constant (Box 2.5).