Antihistamines, available without prescription in many countries, are generally considered to be safe medications; however, the old first-generation H1 antagonists commonly cause adverse central nervous system (CNS) effects, even when administered in usual doses. Patients may not be aware of these effects and do not necessarily develop tolerance to them. In contrast, the new, second-generation H1 antagonists are relatively free from adverse effects in the CNS, primarily because they do not cross the blood–brain barrier and block the important neurotransmitter function of histamine. Most o fthe H1 antagonists in current use are unlikely to cause cardiac toxicity. There is no evidence that H1 antagonists, which have been approved by regulatory agencies, have carcinogenic, tumour-promoting, or teratogenic effects in humans.