I've had no problems with generics. My husband has thyroid issues & he does notice a difference between Synthroid and levoxyl.

It can be the filler inactive ingredients that differ that cause issues for some people. My husband is very sensitive to medications to start with such as pain relievers he had to take after back surgery. One time he took one capsule of my muscle relaxer (tizanidine) and got extremely dizzy and exhausted, but I can take up to 4 in a day. Even if I took 4 of the tizanidine at once, it would probably give me a 50/50 shot at sleep. He is a foot taller than I am and nearly 100 lb. more in weight.