Plasma concentration versus time: o= rapid absorption ³ o x= normal absorption ³ o o += slow release Toxic ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ³ o xo Conc. ³ x x ³ o o x Therap'ticÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ³ x o + x + ³ o + x + ³ x + o x + ³ + x ³o x+ o ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ Time
In absorption, the plasma concentration reaches a peak and then falls away to nothing. The curve depends on the rate and extent of absorption. Thus, in rapid absorption the peak may be very high, such that this plasma concentration may be toxic. In some instances, such as GTN in angina, the side effects from very rapid absorption are acceptable. In slow absorption the rate of release may be sufficiently slow that a detectable plasma concentration is never reached. Importantly though, the total area under curve is the same - i.e. the extent of absorption is the same.
Disease may alter the profile - e.g. analgesics in migraine - such that therapeutic concentration is not reached. In this instance other drugs, such as metaclopramide, may be used to increase gastric opening, and thus promote the absorption of aspirin or paracetamol.
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