H1-antihistamines are inverse agonists that combine with and stabilize inactive conformation of H1-receptors. Thus they interfere with actions of histamine at H1-receptors. They are widely used for treatment of allergic rhinitis, allergic conjunctivitis, urticaria, coughs, colds and insomnia. H1-antihistamines are classified as older 'first generation' and newer 'second generation'. First generation H1-antihistamines have poor receptor H1-receptor selectivity, and cross blood-brain-barrier. They have a lot of adverse events such as anti-muscarinic, anti-α-adrenergic, anti-serotonin, and sedative effects. In contrast, second generation H1-antihistamines were highly selective for the histamine H1-receptor, do not cross the blood brain barrier, and have minimal adverse events. The risks of first-generation H1-antihistamines have been clearly underestimated, particularly when purchased as nonprescribed over the counter medications by public. This review summarizes current literature to evaluate antihistamines including their mechanism, indications and side-effects.