Cytochrome P450 1A2 is an important drug metabolizing enzyme in the liver that metabolizes many commonly used drugs including theophylline, imipramine, propranolol, and clozapine.   CYP1A2 is significantly induced in a clinically relevant manner by tobacco smoking. The clearance of theophylline, imipramine, propranolol and clozapine are all increased by smoking.  Thus, people who smoke may require higher doses of some medications that are substrates of CYP1A2.  In contrast,  a smoker would require a decrease  in theophylline dosage if, for example, smoking were discontinued and the enzyme no longer induced.  This topic  has been recently reviewed by Zevin and Benowitz.{Zevin}

 Inhibitors of CYP1A2,  including some fluoroquinolone antibiotics, can increase the plasma concentrations of drugs that are metabolized by CYP1A2, with a potential for increased toxicity.{Raaska}{Grasela}
Zevin S, Benowitz NL. Drug interactions with tobacco smoking. An update. Clin Pharmacokinet 1999; 36(6):425-438.
Raaska K, Neuvonen PJ. Ciprofloxacin increases serum clozapine and N-desmethylclozapine: a study in patients with schizophrenia. Eur J Clin Pharmacol 2000; 56(8):585-589.
 Grasela TH, Jr., Dreis MW. An evaluation of the quinolone-theophylline interaction using the Food and Drug Administration spontaneous reporting system. Arch Intern Med 1992; 152(3):617-621.