All drugs are potentially poisonous to humans as well as animals. Toxicity may be mild or severe and includes carcinogenic and teratogenic effects. Warnings are given in the monographs. However, risks to humans dispensing medicines are not always well characterized and idiosyncratic reactions may occur. It is good practice for everyone to wear protective clothing (including gloves) when directly handling medicines, not to eat or drink (or store food or drink) near medicines, and to wash their hands frequently when working with medicines. Gloves, masks and safety glasses should be worn if handling potentially toxic liquids, powders or broken tablets. Do not break tablets of antineoplastic cytotoxic drugs, and use laminar flow cabinets for the preparation and dispensing of these medications (Appendix on Safety and Handling of Chemotherapeutic agents).

Many prescribers and users of medicines are not aware of the carcinogenic potential of the drugs they are handling. Below are lists of medicines included in the BSAVA Formulary that are known or potential carcinogens or teratogens. The lists are not all-inclusive: they include only those substances that have been evaluated. Most of the drugs are connected only with certain kinds of cancer. The relative carcinogenicity of the agents varies considerably and some do not cause cancer at all times or under all circumstances. Some may only be carcinogenic or teratogenic if a person is exposed in a certain way (for example, ingesting as opposed to touching the drug). For more detailed information refer to the International Agency for Research on Cancer (IARC) or the National Toxicology Program (NTP) (information is available on their respective websites).

Examples of drugs known or suspected to be human carcinogens (c) or teratogens (t):

• ACE inhibitors (t), e.g. benazepril, enalapril
• Androgenic (anabolic) steroids (t, c)
• Antibiotics (c), e.g. metronidazole, chloramphenicol
• Antibiotics (t) e.g. aminoglycosides, doxycycline, trimethoprim, sulphonamides
• Antifungals (c), e.g. fluconazole, itraconazole
• Antineoplastic drugs (c, t) – all
• Antithyroid drugs (t), e.g. carbimazole/methimazole
• Beta-blockers (t)
• Deferoxamine (t)
• Diltiazem (t)
• Finasteride (t)
• Immunosuppressives (c), e.g. azathioprine, ciclosporin
• Methotrexate (t)
• Misoprostol (t)
• NSAIDs (t)
• Penicillamine (t)
• Phenoxybenzamine (c)
• Progestagens (c) and some oestrogens (c)
• Vitamin A (t)

Note that most carcinogens are also likely to be teratogens.