Atenolol does have side effects and sensitive animals may pass out.

You can read about atenolol here

http://www.petplace.com/drug-library/atenolol-tenormin/page1.aspx

scrolling down for all the info and data on side effects etc.

I'd call your vet about the reaction.

You can read about Tapazole here

http://www.petplace.com/drug-library/methimazole-tapazole/page1.aspx

Hope this helps you!

It could drop the blood pressure too low.

Also the atenolol does not control the thyroid level just the hypertension that can go with it and thyroid imbalance can trigger seizures.

It could be the combination of changes with your cat is an issue.

The hypertension that thyroid problems can create needs to be treated to avoid sudden blindness o rother health issues.

You can read about hypertension here

http://www.marvistavet.com/html/high_blood_pressure.html

http://www.sniksnak.com/cathealth/hypertension.html

They replaced the thyroid med with one to control blood pressure - its the rise in blood pressure that causes the eye problems with the retina.

I do think you should be working with your vet on which meds you are using and letting them know about reactions you are seeing. There are alternative meds for the blood pressure control. You need to control the thyroid issue and the blood pressure issue both and deal with the cause of the anemia too.

Tapazole can trigger aplastic anemia

http://www.annals.org/cgi/content/abstract/41/4/844

http://www.petcarerx.com/pcrx/HealthGuide/Medications/Medication.aspx?mid=10783

but its not a common reaction to the med - but it may be a reaction your cat does have.