Paper I
2018 February (2010 Scheme) · 40 marks · 120 min
Question
Plasma half life
Q33 marksShort Notes
Answer
Plasma half-life (t½) is the time taken for the plasma concentration of a drug to fall to half its original value.
First-order kinetics
- t½ = 0.693/Ke; constant regardless of dose/concentration
- After ~4–5 half-lives, drug is essentially eliminated and steady state on repeated dosing is achieved
Clinical significance
- Determines dosing frequency/interval
- Determines time to reach steady state
- Determines time for a drug to be essentially eliminated after stopping it
- In zero-order kinetics (e.g., phenytoin, ethanol), half-life is not constant — it increases with dose
Related Concepts
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