NSAIDs Phenazone

The second group of non-opioid analgesics, which are not classified as NSAIDs, consists of substances that lack anti-inflammatory properties, such as phenazones, metamizole (dipyrone) and paracetamol. Their molecules are neutral or weakly basic, have no hydrophilic... 

The probable reason is that the phenobarbital increases the metabolism of these NSAIDs by the liver, thereby hastening their clearance. Phenazone is metabolised by mixed function oxidase enzymes in the liver, for which reason it is extensively used as a model drug for studying whether other drugs induce or inhibit liver enzymes. In one study phenobarbital caused about a 40% reduction thereby demonstrating that the liver enzymes were being stimulated to metabolise the phenazone more rapidly. The clinical importance of these interactions is uncertain (probably small) but be alert for any evidence of reduced NSAID effects if phenobarbital is added. 

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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