Adverse effects, like intended effects of drugs, are a function of dosage or drug levels at the target organs, so they may be avoided or decreased by means of careful and precise pharmacodynamics (the change of drug levels in the organism in function of time after administration).

Adverse effects may also be caused by drug interaction, i.e., when physicians fail to check for all medicaments a patient is taking and prescribe new ones which interact agonistically or antagonistically (potentiate or decrease the intended therapeutic effect). Significant morbidity and mortality is caused around the world because of this. Drug-drug and food-drug interactions may occur, and even so-called "natural drugs" used in alternative medicine may have dangerous adverse effects. For example, extracts of St. John's wort (Hypericum perforatum), a phytotherapic used for treating mild depression are known to cause an increase in the cytochrome P450 enzymes responsible for the metabolism and elimination of many drugs, so that patients taking it are likely to experience a reduction in blood levels of drugs that they are taking for other purposes, such as cancer chemotherapeutic drugs, protease inhibitors for HIV and oral contraceptives.

The scientific field of activity associated with drug safety is increasingly government-regulated and is of major concern for the public as well as to drug manufacturers. The distinction between adverse and non-adverse effects is a major undertaking when a new drug is developed and tested before marketing it. This is done in toxicity studies to determine the non-adverse effect level (NOAEL). These studies are used to define the dosage to be used in human testing (phase I) as well as to calculate the maximum admissible daily intake. Imperfections in clinical trials, such as insufficient number of patients or short duration, sometimes lead to public health disasters such as those of fenfluramine (the so-called fen-phen episode), thalidomide and, more recently, of cerivastatin (Baycol®, Lipobay®) and rofecoxib (Vioxx®), where drastic adverse effects were observed, like teratogenesis, pulmonary hypertension, stroke, heart disease, neuropathy, etc., and a significant number of deaths, causing the forced or voluntary withdrawal of the drug from the market.

Most drugs have a large list of non-severe or mild adverse effects which do not rule out the interruption of usage. These effects have widely variable incidence, according to individual sensitivity. They comprise nausea, dizziness, diarrhea, malaise, vomit, headache, dermatitis, dry mouth, etc.

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