← Glossary

Half-life

See also: Pharmacokinetics (PK)Bioavailability

Half-life is the time it takes for a substance’s concentration (or amount in the body) to decrease by 50% under defined conditions. It is a fundamental pharmacokinetic parameter used to characterize how quickly a substance is eliminated from the body.

Key Concepts

  • Elimination Half-life: The time required for the plasma concentration of a substance to decrease by half through metabolism and excretion
  • Dose Independence: For most substances, half-life remains relatively constant regardless of dose (within therapeutic ranges)
  • Multiple Half-lives: After approximately 5-7 half-lives, a substance is typically considered to be nearly eliminated from the body

Determinants of Half-life

Half-life is influenced by several factors:

  • Clearance: The rate at which the body eliminates the substance (primarily through metabolism and excretion)
  • Volume of Distribution: The theoretical volume into which the substance distributes in the body
  • Absorption Rate: For substances entering through routes other than intravenous injection

The relationship is expressed as: Half-life = (0.693 × Volume of Distribution) / Clearance

Peptide-Specific Considerations

In peptide research, half-life is frequently studied because:

  • Short half-lives are common for many natural peptides due to rapid enzymatic degradation
  • Modifications (such as PEGylation, amino acid substitutions, or fatty acid conjugation) can extend half-life
  • Route of administration significantly affects half-life (subcutaneous vs. intravenous vs. oral)
  • Individual variation can be substantial due to differences in metabolic enzymes

Clinical vs. Research Context

Half-life values reported in research settings may differ from clinical use due to:

  • Purity and formulation differences
  • Species-specific metabolism (animal models vs. humans)
  • Route of administration
  • Co-administered substances or dietary factors

Note: Half-life is a population-level average and significant individual variation exists. It describes pharmacokinetics, not pharmacology—it tells us how long a substance remains in the system, not what effects it produces.