Week 1- You should start medications earlier in the year than you may think.
If you are your child has seasonal allergies, you should ideally start the medicines at least two to three weeks before the beginning of the season ( which depends on his specific allergy) and continue them through the end. If you have allergies year-round, talk to your doctor about keeping the medicines going all year rather than waiting for symptoms to flare up.
Most allergy medicines are far more effective at preventing symptoms than they are at treating them once they start. That's because the runny nose, stuffiness, itching, sneezing and watery eyes result from a whole cascade of events in the immune system, and once it gets going it's hard to stop. Corticosteriod nose sprays, available over-the-counter, are the most effective type of medicines for allergies (ages 2 and older); they prevent the cascade by decreasing the immune system's response to antigens like dust mites and pollen.
Antihistamines can help relieve symptoms also. They are more effective if they are in bloodstream before the allergy attack. They need about 5 days to reach their full strength and about 5 days to wear off completely.