Apparently, methimazole is only designed to be used very short term. Results will show at about 18 months. If they don’t show up by 18 months, chances are you aren’t going to see promising results. The success rate is less than 50%, with some sources citing only a 20% or 30% success rate. I was on it for 22 months. My thyroid hormone finally got to normal. I got off the drug, and my thyroid has started to increase its hormones. My Grave’s Disease is back.