1/2 to 1 hour. Some antihistamines require metabolism in the liver to be converted into the most effective form to block histamine. These antihistamines take at least 1 hour to become effective. However, some antihistamines like allegra, xyzal (levocetirizine) and Clarinex are already in the active form and begin to work as soon as they are absorbed which take about 30 minutes.
About 1 hour. Most of the newer antihistamines work within the first hour of taking them. About 1/3 of people with mild to moderate allergies have some relief. Avoidance measures are important. Allergy shots are effective in 85% of properly selected patients who have moderate to severe allergies to inhalant allergens.
20min to 1h. Most oral antihistamines can start to take effect as early as 20 minutes and are usually in full effect by 1 hour. However, the old adage of "an ounce of prevention is worth a pound of cure" still holds true. Taking a pill before entering an allergic environment can control symptoms better than taking a pill once the symptoms have already started. Plan ahead when possible.