Rx Tapazol (Methimazole) compounded-advice?

My 11 yr old kitty baby just diagnosed with hyperthyroidism. Prescribed Tapazol (Methimazole) which I had compounded into a topical gel to be applied to ear (to make it easier for kitty & me!) I'm worried about side effects (nausea, lethargy, etc.) Any advice? What about something holistic, such as Resthyro? Please, please advise. Thank you SO much.