CYP P450 induction
Some drugs are able to increase the activity of the microsomal enzymes and thus increase the clearing ability of the liver. These are called inducers. As shown in the graphic, when an inducing drug is added to steady-state concentrations of a drug which is a substrate, this leads to a reversible increase in enzyme activity resulting in a decrease in the substrates serum concentrations.
The effects of enzyme induction can been typically be seen after the first 2 days, but maximal effects may take more a week.
Look at the clinical example shown in the table where a patient is taking a daily dose of a coumarin in order to lower the risk of a DVT. This patient then gets prescribed rifampin (a known enzyme inducer). It is possible that the serum concentration of the coumarin will be decreased to the point where the patient is no longer receiving adequate anticoagulation and is not protected from getting a DVT.
See how an inducer effects the concentration of another drug by clicking here and selecting "add inducer".