I want a cardiac ct scan but am scared of allergic reaction to the dye, i have mild asthma. can i request benadryl (diphenhydramine) or how can i get round this?

Why Cardiac CT?: Why at 21 yo is a cardiac ct desired? Risk of adverse reaction to contrast is less than 1%, and serious adverse reaction is about 1-2 in 10, 000 injections.

See answer: Talk to your dr. / radiologist about the appropriate pre- procedure allergic preparation. It usually involves pre-procedure oral steroids (if you are an asthmatic already on steroids this may be modified), possibly an oral antihistamine, and may be performed in a hospital center depending on the severity of your prior iodine sensitivity.