Phenazone Warfarin

The phenazone component of dichloralphenazone is an enzyme inducer and hastens the metabolism of warfarin (4). 

Breckenridge A, Orme ML, Thorgeirsson S, Davies DS, Brooks RV. Drug interactions with warfarin studies with dichloralphenazone, chloral hydrate and phenazone (anti-pyrine). Clin Sci 1971 40(4) 351-64. 

The phenazone , (p.434) component of dichloralphenazone is a potent liver enzyme inducer, which increases the metabolism and clearance of the warfarin, thereby reducing its effects. The effects of the cloral hydrate , (p.396) component appear to be minimal. 

The anticoagulant effects of warfarin are reduced by phenazone. Clinical evidence... 

The plasma warfarin concentrations were halved (from 2.93 to 1.41 micrograms/mL) and the anticoagulant effects accordingly reduced after 5 patients took phenazone 600 mg daily for 50 days. The throm-botest percentage of one patient rose from 5 to 50%. In an associated study it was found that phenazone 600 mg daily for 30 days caused a reduction in the warfarin half-live from 47 to 27 hours and from 69 to 39 hours, respectively, in 2 patients. ... 

Phenazone is an enzyme inducer, which increases the metabolism and clearance of warfarin, thereby reducing its effects. ... 

An established interaction. The effects of concurrent use should be monitored and the dosage of warfarin increased appropriately. However, note that phenazone is little used clinically, and an alternative NSAID ,... 

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