![]() 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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