Half Life Uses in Medicine and Pharmacology

Drug Dosing Intervals

Drugs are usually dosed at roughly one to two half lives, so a drug with a six hour half life is taken every six to twelve hours. A drug with a thirty hour half life supports once daily dosing. This is why a single daily tablet can maintain a steady blood level while an equivalent drug must be taken four times a day.

The Washout Period

To clear a drug from the body takes roughly five half lives, by which point under 3 percent remains. This is the standard rule before surgery, before pregnancy, and before switching to a similar drug that would otherwise accumulate.

Imaging Tracers

Technetium 99m, used in a large share of nuclear medicine scans, has a half life of about six hours. That is long enough to complete a scan and short enough that the patient is clear well before leaving, and it is the reason such tracers can be administered in routine outpatient settings.

Why Half Lives Are Kept Short

A medical tracer must deliver useful counts quickly while disappearing quickly afterwards. A longer half life would mean a larger dose for the same image quality, because more activity would be needed to survive the imaging window.

Accumulation With Repeated Doses

With repeated dosing at intervals shorter than the half life, the drug accumulates until each dose roughly replaces what was lost. Steady state is typically reached after about four to five half lives of consistent dosing.

Reading a Half Life on a Label

Half lives vary enormously, from minutes for some anaesthetics to days for some psychiatric medications. Always check whether the figure quoted is the elimination half life from the body or the biological half life of the active compound, which are not always the same.