Pocket Psych Drugs 2nd Edition PDF – Point-of-Care Clinical Guide
Pharmacokinetics (PK) can be defined as “how the body processes a drug” resulting in a drug’s concentration in the body. This is done through absorption, distribution, metabolism, and excretion (ADME). Absorption: Describes the drug’s movement from point of administration (oral, injection, skin) until it reaches the bloodstream. In oral administration, first-pass metabolism refers to how much of the drug is metabolized by the liver and therefore is not available to the bloodstream (bioavailability of drug). This determines the dose needed for oral administration or the need for an alternative route of entry (such as parenteral). Distribution: Movement of drug from the bloodstream to the rest of the body. Concerned with movement over the blood-brain barrier (may affect the brain) or crossing the placenta (may affect the fetus).
Also concerns highly protein-bound drugs that may cause drug interactions. Metabolism and Excretion: The primary organ of metabolism is the liver, and excretion of drugs takes place through the kidneys. Dosing considerations are based on how well the liver and kidneys are functioning. Half-life is also a factor as drugs with long half-lives may accumulate, resulting in overdose or toxicity. Half-life is the time (hours) that it takes for 50% of a drug to be eliminated from the body. Time to total elimination involves halving the remaining 50%, and so forth, until total elimination. Half-life is considered in determining dosing frequency and in determining time to steady state. The rule of thumb for steady state (stable concentration/manufacture effect) attainment is 4–5 half-lives. Because of fluoxetine’s long half-life, a 5-week washout is recommended after stopping fluoxetine and before starting an MAOI to avoid a serious and possibly fatal reaction.