Can Invisalign be Used if You Have Bad Allergies?

While ANYTHING is possible, the likelihood that a person is allergic to the plastic in the aligners is rare.  If they are, then orthodontics in any form is not likely possible, as the glue for brackets would be worse.

As far as OTHER allergies, Invisalign shouldn't affect them.  The aligners are very thin, so if airway was an issue due to hayfever or the like, Invisalign won't make them worse.

Invisalign will not interfere with breathing, so if you're worried about having asthma or stuffy nose from common allergies Invisalign will not make breathing more difficult.  There have been rare cases of allergy to the material used in Invisalign so if you're worried about that you should visit your orthodontist and ask about it.  If you have known allergies to any form of plastic you might want to be tested for the type of plastic used in Invisalign before spending the money to get treated with it.  Just for comparison, allergies to metal and latex which are often used in conventional braces are probably more common than allergies to the plastic used in Invisalign.