Which of the following drugs is known to REDUCE the effect of warfarin?

Rifampicin is known to reduce the effect of warfarin due to its potent enzyme-inducing properties. It acts primarily as an inducer of the cytochrome P450 enzyme system, particularly CYP2C9 and CYP3A4, which are responsible for the metabolism of warfarin. By increasing the metabolism of warfarin, rifampicin causes lower plasma levels of warfarin, leading to a decreased anticoagulant effect.

In contrast, azathioprine and methylphenidate do not significantly interact with the warfarin metabolism to a degree that would reliably reduce its efficacy. Additionally, allopurinol is primarily involved in the metabolism of purines and does not have a notable effect on warfarin levels. Therefore, among the options provided, rifampicin stands out as the drug that significantly decreases the effect of warfarin through its action as an enzyme inducer.