Are you asking about whether you'll flare or whether you might have side effects? For flares, it would be the same as not ever taking the drug because it doesn't stay in your system. Once you're off, you have the same chances of having flares as you did before.
The risks include some form of cancer in a TINY percentage of people and there's even a debate about whether it's a significant risk that any one should bother considering. My guess is that it's dose related. But I don't know. I would assume that the shorter you were on it, the lower your risk. It's small, really. In my mind, the risk of bleeding every day is more pressing.
The reactions include liver problems... which is why you need to have your blood tested regularly when you're on it. Once you're off the drug, any 6mp-induced liver problems would only persist if the liver problem was some kind of serious thing that wasn't caught early enough. You can confirm that you're fine after stopping 6MP with a blood test for liver function.
Another response to 6MP is lowered white blood count because it's an immunosuppressor. Again, blood tests can tell you how you are doing once you're off the drug.
Just curious -- do you recall where you read what you did about adverse reactions years later? I doubt it's something like an acid flashback. Can you imagine a 6MP flashback? Ooh, strange.
UC for the last ten years
Current Meds: 6MP since 2006, Cortifoam
Past Meds: You name it; I've tried it. (Asacol, Colazol, Pentasa, DiPentum, Rowasa, Canasa, Cortenema, sulfasalazine)
5ASA drugs don't work for me. Canasa seems to make me worse.