Rifampin induces liver microsomal enzymes, thereby decreasing the potency of other drugs metabolized by this system.
For example, rifampin decreases the effect of oral contraceptives, which can lead to unwanted pregnancies. It also decreases the potency of antiepileptic drugs, which can lead to seizures. NNRTI treatment in HIV patients is also negatively impacted.
ALso remember the other classic examples such as theophylline, warfarin, and phenytoin.
Because these have a narrow therapeutic index, a decrease in their plasma concentration can more easily lead to therapeutic failure (just as an increase can more easily lead to toxicity)