An adverse drug reaction may be defined as "an appreciably harmful or unpleasant reaction, resulting from an intervention related to the use of a medicinal product, which predicts hazard from future administration and warrants prevention or specific treatment, or alteration of the dosage regimen, or withdrawal of the product'' (Edwards and Aronson, 2000). This has to be contrasted with the term adverse drug event, which refers to untoward occurrences following drug exposure but not necessarily caused by the medicine (Asscher et al., 1995). This chapter focuses on adverse drug reactions rather than adverse events.
While the drug discovery process has been revolutionized by new techniques such as combinatorial chemistry and high-throughput screening, drug safety assessment lags well behind and is still reliant on many of the same technologies that have been used for several decades. By the time a drug is marketed, only about 1500 patients may have been exposed to the drug (Asscher et al., 1995; Rawlins, 1995). Thus, only those adverse reactions occurring at a frequency of greater than 1 in 500 will have been identified at the time of licensing. Assessment of adverse drug reactions therefore is likely to represent an important aspect of drug therapy for many years to come, and indeed, with the development of new biotechnology compounds, it is likely that the pattern of these reactions will change. Furthermore, through the use of gene and protein screening technologies, many new targets will be discovered. As new drugs are developed to modulate the function of these targets, it is very unlikely that we will fully understand the biology of the new target molecule(s), and this will lead to unforeseen adverse reactions. For example, adverse effects such as exacerbation of multiple sclerosis, systemic lupus erythematosus (SLE) and blood dyscrasias that are being reported with anti-TNF therapies (Sharief and Hentges, 1991; Furst et al., 2000) would not have been expected given that TNF-a is involved in their pathogenesis.
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