I have allergies and asthma. will reactine help with asthma. i take 10mg but sometimes i think it will help more so i take 20mg. is it useful?

Maybe for allergies: Cetirizine (the generic name) is an antihistamine, and these meds are very good and controlling symptoms like sneezing, itchy eyes, or an itchy nose. In most cases, 10 mg will be enough to help these symptoms, but in certain cases your doctor may recommend higher doses. Antihistamines do not treat the inflammation that causes asthma. Talk with your doctor about your symptoms and medication regimen.

Not much: Reactine (cetirizine or zyrtec) is an antihistamine and has little effect on the lungs. There may be very minimal improvement in lung function, and controlling nasal allergies certainly can help with asthma. I doubt you would see much difference by increasing the dosage. Higher doses of this medication are often use in europe for hives.