Is not metabolized (no biotransformation).Is not distributed to tissues (stays in plasma).Acidic drugs bind albumin while basic drugs mostly bind globulins.Įxample includes warfarin, an anticoagulant, having very long half life due to extensive protein binding. Drugs having plasma protein binding have longer half life ( directly proportional). Protein bound drug produces no effect and is not excreted because proteins are not filtered by glomeruli. Plasma half life of some drugs: Drugįactors affecting Half Life: 1. Elimination half life is about 7 to 10 hours. Once it leaves the brain, person regains consciousness but because it is deposited in the tissues, person keeps on feeling drowsy. After administration, the drug immediately goes to the brain, produces its effects, then leaves plasma to get deposited in the tissues. It is administered through intravenous route and has very short duration of action (about 5 to 10 min). Thiopentone sodium us used as a general anesthetic. More the clearance of a drug, shorter is the half life. More time is required for their elimination, thus have greater half life.
Some drugs have high volume of distribution and are distributed to various tissues, mostly adipose tissue. When the drug is absorbed and reaches the plasma, it is distributed to the tissues. Drugs having beta half life have two half lives, one in the plasma and one in the tissues. Most of the drugs have alpha half life and remain in the plasma. Half life = 0.693 x Vd/ total body clearanceĪlpha half life = plasma / distribution half lifeīeta half life = tissue / elimination half life Half life of a drug is directly proportional to the volume of the distribution and inversely proportional to the clearance. “Time required for the concentration of a drug to fall to half of its initial concentration after reaching its peak.”įor example, after intravenous administration, if maximum concentration is 16 mg and the half life is 2 hours, after 2 hours 8 mg will be left, and so on. Once the level falls below the minimum effective plasma concentration, the effect is over. Drug requires a minimum effective plasma concentration for the effects to start appearing. In case of oral route, plasma concentration gradually rises and reaches its peak value, then it begins to fall down.
Onset, duration and intensity of effect of drug depends on the rate of absorption, distribution and elimination (biotransformation, excretion).
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