I've had some trouble. My employer provided insurance denies it - I have a state-funded insurance plan for people with genetic diseases and they cover the cost (but only after denial from my primary insurance). Luckily, my pharmacy handles all of that so it only really affects me every 6 months or so when the pharmacy has to get a new denial letter and it ends up taking two weeks for a refill instead of two days. And whereas I have a co-payment for all the drugs my primary does pay for, anything they deny that ends up going to the state insurance I get for free - no co-pay required.