Drug Half Life

Figures A.6 to A.8 demonstrate how drug levels in plasma go down after administration of a single dose. An important indicator of the duration of a drug's effects is the time required for its concentration in blood to decline by half. This is frequently a constant interval independent of the actual concentration at a given moment. Immediately after absorption, plasma levels of a drug decline faster because the drug is distributed into tissues. Thereafter elimination processes are responsible for the slower reduction

Figure A.8. Plasma levels of a drug after administration of a single oral dose (500 mg) in six volunteers. On abscissa, time after administration. On ordinate, plasma levels of the drug. (Courtesy of Dr. Rein Pahkla, Department of Pharmacology, Tartu University.)

in plasma levels. Because we need to be able to predict how long a drug effect will last and when the organism will be free of the substance, elimination half-life is one of the most important variables in pharmacology.

If a drug has short elimination half-life, it must be taken several times per day, which diminishes the apparent ease of oral administration and reduces patient compliance. Such drugs may be formulated into sustained- or slow-release tablets or capsules (e.g., venlafaxine or bupropion).

Anxiety and Depression 101

Anxiety and Depression 101

Everything you ever wanted to know about. We have been discussing depression and anxiety and how different information that is out on the market only seems to target one particular cure for these two common conditions that seem to walk hand in hand.

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