Antihistamines

Histamine is one of the substances released during an allergic reaction. Antihistamines block the effects of histamine and prevent allergy symptoms. This class of drugs includes older drugs such as chlorpheniramine and antazoline as well as newer ones such as levocarbastine and emedastine. Chlorpheniramine and antazoline are available in combination with vasoconstrictors. The newer drugs are more effective than the older ones. Antihistamines may cause side effects like headache, eye discomfort, stinging of the eyes and cold symptoms.

Oral Antihistamines

Oral antihistamines are useful when nasal or other allergy symptoms are also present. Newer drugs such as levocetirizine, desloratidine, rupatadine, fexofenadine and mizolastine are more effective than the older oral antihistamines. However, they produce dryness of eye as a side effect.

Mast cell stabilizers

Histamine and other substances that cause inflammation are released from certain cells called mast cells. A class of drugs called mast cell stabilizers blocks the release of histamine from these cells. They are most effective in preventing an allergic reaction, hence they should preferably be used when the patient anticipates an allergy. These include cromoglycate, lodoxamine and nedocromil. Lodoxamine(Alomide) is more effective than cromolyn sodium and is useful in severe cases. Nedocromil may be administered twice a day in seasonal allergies, whereas the others are usually administered 4 times a day. They may cause side effects such as headache, burning/stinging or eyes or unpleasant taste.

Antihistamines plus mast cell stabilizers

Some newer antihistamines have additional mast cell stabilizing action. These include azelastine (Optivar, Oculast), epinastine (Elestat), ketotifen (Zaditor eye drops, Alaway eye drops) and olopatadine (Patanol eye drops). Since these drugs act through multiple mechanisms, they may be more effective than plain antihistamines or mast cell stabilizers. They are administered twice a day. They cause side effects like burning/stinging in the eyes, headache or bitter taste. They may also cause symptoms of upper respiratory tract infection. Ketotifen is available as a preservative free preparation. Olopatadine is also available in higher concentration (Pataday eye drops) that is used once a day.