In pharmacology, a dirty drug is an informal term for drugs that may bind to many different molecular targets or receptors in the body, and so tend to have a wide range of effects and possibly adverse drug reactions. Today, pharmaceutical companies try to make new drugs as selective as possible to minimise binding to antitargets and hence reduce the occurrence of side effects and risk of adverse reactions.
Examples of compounds often cited as "dirty drugs" include chlorpromazine, dextromethorphan and ibogaine, all of which bind to multiple receptors or influence multiple receptor systems. There may be instances of advantages to drugs that exhibit multi-receptor activity such as the anti-addictive drug ibogaine that acts within a broad range of neurohormonal systems where activity is also exhibited by drugs commonly associated with addiction including opioids, nicotine, and alcohol. Similarly chlorpromazine is primarily used as an antipsychotic, but its strong serotonin receptor blocking effects make it useful for treating serotonergic crisis such as serotonin syndrome. Dextromethorphan for its part is widely used as a cough medication, but its other actions have led to trials for several conditions such as its use as an adjunct to analgesia, and a potential anti-addictive drug, as well as its occasional recreational use as a dissociative.
Kanamycin is an aminoglycoside antibiotic which induces deafness through blockage of the Outer Hair Cells of the cochlea; yet it has many other effects, weakening for instance the collagen and DNA biosynthesis. It acts by inhibiting the synthesis of proteins in susceptible organisms. Kanamycin requires close clinical supervision because of its potential toxicity and adverse side effects to the auditory and vestibular branches of the eighth cranial nerve and to the renal tubules.
Clozapine and dimebon are examples of drugs used in the treatment of CNS disorders that have a superior efficacy precisely because of their "multifarious" broadspectrum mode of activity. Likewise, in cancer chemotherapeutics, it has been recognized that drugs active at more than one target have a higher probability of being efficacious.
In the field of drugs used to treat depression, the nonselective MAOIs and the TCAs are widely believed to have an efficacy that is superior to the SSRIs. Why then are the SSRIs normally picked as the first-line choice of agent and not the (non-selective) MAOIs and TCAs? The answer to this question is simply based on the notion of SSRI use acting in a manner conducive to higher safety profiles than the nonselective MAOIs and TCAs – This is both in terms of there being less chance of death in the event of overdose (be it intentional or accidental poisoning), but also less risk in terms of dietary restrictions (in the case of the nonselective MAOIs), and also greatly reduced risk of hepatotoxicity (MAOIs) and cardiotoxicity (TCAs). It has also been said that SSRIs are better tolerated than the earlier agents, but SSRIs are not without their side effects.
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