Antihistamine pills often begin to act after 30 minutes of administration and are most effective within 1-2 hours. Antihistamines are more effective when used on a regular basis as a prophylactic precaution before symptoms appear, rather than merely when symptoms develop. Using anthistamines regularly can reduce the severity of allergic reactions to foods. The effect of antihistamines lasts for several hours after they are administered orally or absorbed through the skin. They work by blocking the action of histamine, which is responsible for causing itching, rashes, and other symptoms associated with allergies. Antihistamines do not kill allergens directly but instead block their reaction with immune cells called mastocytes. This prevents them from releasing chemical mediators that cause allergic reactions. Antihistamines are used to treat allergies, asthma, chronic urticaria (hives), insect bites and stings, and certain other conditions. They may also be used as a sleep aid by some people who suffer from insomnia. Antihistamines are available as tablets, capsules, liquids, syrups, sprays, patches, and intranasal formulations. Some antihistamines have been found to be toxic if taken in large doses over a long period of time. These include diphenhydramine (Benadryl) and chlorpheniramine (Chlor-Trimeton).