Cyclophosphamide (INN), also known as cytophosphane, is a medication mainly used in chemotherapy. It is an alkylating agent of the nitrogen mustard type (specifically, the oxazaphosphorine group).

An alkylating agent adds an alkyl group to DNA. It attaches the alkyl group to the guanine base of DNA, at the number 7 nitrogen atom of the imidazole ring. This interferes with DNA replication by forming intrastrand and interstrand DNA crosslinks.