Applied Pharmacology 4, Half Life of Drugs

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Understanding half life is important to inform how and when we administer any medication. These are not casual viewing, but are for serious students who want to understand the essential underpinning concepts of the care we give our patients.

Notes
Half life t 1/2

Pharmacokinetics and Pharmacodynamics

Bioavailability – the proportion of an administered drug that reaches the systemic circulation in unchanged form

Half life is the time from peak plasma concentration until half is eliminated

Clearance

T ½ First order kinetics
First order elimination kinetics : "Elimination of a constant fraction per time unit of the drug quantity present. The elimination is proportional to the drug concentration."

Zero-order elimination kinetics : The plasma concentration – time profile during the elimination phase is linear (Fig. 1). For example 20 mg are eliminated every hour, independently of the drug concentration in the body. Order 0 elimination is rather less common, mostly occurring when the elimination system is saturated. Eg.
Alcohol
Phenytoin
Warfarin (Coumadin)
Heparin
Paracetamol (acetaminophen)
Over dose of aspirin (normally t ½ = 3-4 hours)

Steady state reached after 5 – 6 half lives (3.3 half lives for 90% concentrations), therefore possible loading dose.

Drug eliminated after 5 half lives (95% eliminated after 4.5 half lives)

Half life t ½ of common drugs

Noradrenaline 2 minutes
Salbutamol 1.6 hours
Morphine 2 – 3 hours
Methadone 24 hours
Gentamycin 2 – 3 hours
Penicillin 6 – 8 hours
Diazepam 24 hours
Fluoxetine 6 days
Warfarin 4 days