In acutely ill patients, many of these parameters will not be known to the provider. An idea of the physiological status of these patients is obtained from their history, physical examination and investigations, allowing us to anticipate the likely response to the drugs prescribed. Most drugs in acute care are administered intravenously and hence can be titrated to effect. This is the most frequently used method to administer the drug until the patients’ needs are met. The adequacy of response may then be determined through further monitoring of the responses expected of the drug as well as development of toxicity from the drug. Monitoring may be assisted by measuring the blood concentrations of the drugs.

Care providers administer drugs with the intention of enhancing the body’s healing process. Most of these processes occur at the cellular or tissue level and hence drugs must reach these sites of action. Many variables determine the amount of drugs that ultimately arrive at these sites to produce the desired effect.

The principles of pharmacology assist the provider in making the most effective use of drugs. Some of these variables that have to be taken into consideration include the recommended dosing regimen, the route of administration, the compliance of the patient to the intake of the drug plus the absorption, distribution, metabolism and excretion of the drug. In addition, several physiological parameters in a patient influence the response to the drug, and these include the genetic make-up of the patient, drug receptor state, drug interactions and, with continued use of the drug, tolerance to the drug or rebound responses on withdrawal of the drug.

Most drugs in the acute care situation are administered intravenously, intramuscularly or subcutaneously. The absorption of most drugs is of course 100 % with the parenteral routes, but absorption of drugs administered orally may be incomplete. Even if the drug is totally absorbed through the gastrointestinal tract, it may undergo biotransformation in the liver before it gets distributed in the systemic circulation. This will result in a reduced systemic bioavailability of the drug (see Chap. 3).