I have a pretty bad allergy to cats and dogs and I also have asthma attacks triggerd by this. I am going over to a friends house today and she owns both a cat and two dogs.  What over the counter medications can I take to prevent an allergy/asthma attack?
Given that asthma attacks can be severe, and occasionally even life-threatening it's probably best to avoid your friends house if possible. If you must go, you should go armed with your asthma reliever puffer (albuterol, albuterol etc.) - note that all puffers should be used through a spacer device or as a turbuhaler. It would also be a good idea to take an anti-histamine tablet. Loratadine usually does the trick and does not cause a lot of Chat Doctor.  If you usually take a preventer puffer (containing steroids) I would double the dose of this for the couple of days before the visit. If despite the above the cats and dogs cause you to feel you really are having an asthma attack you should seek medical advice immediately.