The problem comes in when pharmacies buy from different manufacturers. They changed the
manufacturer of my T3 med, once and I went downhill in a hurry. I had tell them, which manufacturer I will accept T3 med from, so they flagged my account accordingly.
As MichyDevon said, each manufacturer uses different fillers/binders to which you could react adversely. If you find one that you do well on, there shouldn't be any reason not to take it, so long as your doctor is okay with it.