My doctor said that benadryl (diphenhydramine) could make an asthma attack worse. but that zyrtec or allegra would be better. why?

Antihistamines dry: Benadryl (diphenhydramine) is more drying than the newer antihistamines in general, and we worry that occasionally an antihistamine will dry secretions so much that they will solidify inside you. This is rare, but has happened. When it first came out, Benadryl (diphenhydramine) was actually a treatment for asthma and one of my patients says they flew it to dallas from st. Louis to treat her asthma! she was 6 mo old & it saved her.

See below: Benadryl (diphenhydramine) should not adversely affect your asthma, in fact if might slightly help as it has a small anticholinergic effect similar to atrovent but no where near as powerful. The other meds mentioned may help as well. I'm not sure where your doctor was going with the opinion but it may be the belief that it may sedate and lower your resp rate and if an attack is occurring - possible make it worse.