Final answer:
Information on discarding expired methotrexate is found in the USP (United States Pharmacopeia), which offers guidelines on safe drug disposal to prevent environmental pollution.
Step-by-step explanation:
Information about discarding expired methotrexate can be found in the USP (United States Pharmacopeia). The USP provides guidelines on the proper disposal of medicines to prevent environmental pollution. The Physician's Desk Reference (PDR) contains drug prescribing information but does not typically provide disposal instructions. The Red Book focuses on pediatric infectious diseases and their treatment, and the Orange Book lists approved drug products with therapeutic equivalence evaluations but does not address drug disposal.
Proper disposal is critical because, as investigations in Germany have shown, a significant percentage of unused medications end up being improperly disposed of via toilets or sinks, which can contribute to environmental pollution. In the United States, drug take-back programs are voluntary and often localized, urging people to not dispose of pharmaceuticals in household waste or sewage systems.