In pharmacology, an antitarget is a receptor, enzyme, or other biological target that is affected by a drug, which causes undesirable side effects. During drug design and development, it is important for pharmaceutical companies to ensure that new drugs do not show significant activity at any of a range of antitargets, most which have been discovered largely through chance.^[1][2]

Among the best known and most significant antitargets are the hERG channel and the 5-HT_2B receptor, both of which cause long-term problems with heart function which can prove fatal (long QT syndrome and cardiac fibrosis respectively), in a small but unpredictable proportion of users. These targets were both discovered as a result of high levels of distinctive side effects during the marketing of certain medicines, and while some older drugs with significant hERG activity are still used with caution, most drugs that have been found to be strong 5-HT_2B agonists were withdrawn from the market, and any new compound will almost always be discontinued from further development if initial screening shows high affinity for these targets.^{[3][4][5][6][7][8]}

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