Hello – Swelling is listed as a possible side effect of methimazole, *but* there can also be other numerous other causes as well.
I would suggest getting in to your doctor to have him/her take a look and give you some further direction. Some patients who have minor allergic reactions to methimazole will do fine with PTU, while others will have the same reaction they did with methimazole.
Of course, swelling can sometimes be something as simple as too much salt, but it’s best to keep your doctor in the loop, just to err on the side of caution!