I had a lot of side effects when I started taking 6MP. From memory I mainly experienced:
Hayfever like symptoms
Severe Tiredness but unable to sleep
Nausea (like car sickness)
General unwellness
My doctor advised that you need to slowly up the doseage over a period of time. In my case, due to these side effects, we had to stop the drug then more than usual very slowly introduce it over a 2 month period.
Once I was on the full 50mg dose I was fine for about 6 months however I started getting more side effects slowly. Mainly a slight nauseated feeling near constantly. My Specialist felt it was not the drug as it usually is ok once you get onto it and the initial side effects subside. After 12 months and a colonoscopy that found the crohns in remission due to the 6MP (probably) I was still feeling poorly. I suspected 6MP. My doc and I agreed to 1/2 the dose and I almost instantly felt better. It actually felt like I had been walking around in a haze for months - like vaseline on my eyes and cotton wool in my head. I had been unable to engage in any meaningful conversations or socialise as I couldn't concentrate - but because the symptoms increased gradually I didn't realise how bad it was until I 1/2'ed the dose.
So in my experience I would recommend:
1. Slowly start to increase the dosage
2. Stick with it if you can bear it for at least 6 weeks
3. After you have been on it for a while but you slowly start feeling poorly again it could be this drug
Good luck!