This chapter discusses antihistamines that are used in combination with a nasal decongestant or with a leukotriene receptor antagonist. One randomized, double-blind, placebo-controlled crossover clinical study in patients with persistent allergic rhinitis showed that montelukast combined with either desloratadine or levocetirizine is more effective in reducing symptoms than monotherapy with these agents. All the treatments were well tolerated. Second-generation antihistamines may also be beneficial in patients with asthma and concomitant rhinitis. Drug-induced movement disorders are most commonly associated with neuroleptic drugs. However, there are a few reports of abnormal involuntary movements associated with antihistamines. A pharmacokinetic study in six healthy men after a single dose of cetirizine 20 mg, pilsicainide 50 mg, or both showed that the renal clearance of each was significantly reduced by co-administration. In vitro studies using Xenopus oocytes with microinjected human organic cation transporter 2 and renal cells transfected with human multidrug resistance protein 1 showed that the transport of the substrates of these transporters was inhibited by both cetirizine and pilsicainide.