Which medication is associated with a metallic taste in the mouth?

Metronidazole is known to be associated with a metallic taste in the mouth, among other potential side effects. This side effect can be particularly pronounced in some individuals and is a well-documented phenomenon in clinical practice. The reason behind this metallic taste is not entirely understood, but it may be related to the way metronidazole interacts with taste receptors or alters the flora of the oral cavity.

Other medications listed, while they may have their own side effects, do not commonly cause a metallic taste as a recognized effect. Amoxicillin, for instance, is typically well-tolerated and does not frequently lead to alterations in taste perception. Doxycycline is primarily an antibiotic used for various infections and is not known for causing metallic taste. Cyclophosphamide, an alkylating agent used in chemotherapy, has a range of side effects, but a metallic taste is not one of the reported common outcomes. Thus, metronidazole stands out as the correct answer due to its specific and recognized association with this particular side effect.