The original drug may take on the role of a pro-drug. This alteration can be an increase by induction of enzyme activity or a reduction by competitive inhibition. A large number of drugs can cause an increase over time in liver enzyme activity. This in turn can increase the metabolic rate of the same or other drugs. Phenobarbitone will induce the metabolism of itself, phenytoin, warfarin, etc.
Cigarette smoking can cause increased elimination of theophylline and other compounds.
Dosing rates may need to be increased to maintain effective plasma concentrations. Alternately some drugs can inhibit the metabolism of other drugs. Drug metabolism being an enzymatic process can be subjected to competitive inhibition. For example, warfarin inhibits tolbutamide elimination which can lead to the accumulation of drug and may require a downward adjustment of dose.