I've been on mercaptopurine for just over a year now (since i was diagnosed). It is scary reading all the side effects. I'm not 100% sure about not being able to get pregnant because I haven't tried! However I was a bit nauseous when I first started, it lasted about a week and once my body adjusted to the medication it was gone. The 2 main things I notice from it is that I get sick a lot, I catch lots of colds and even though I used to before being diagnosed, it takes a lot longer to feel better.
The other thing I noticed was hair loss, which really really scared me. My hair would just fall out when I brushed my fingers through it, let alone a brush. And it also stopped growing longer. It took months for that to stop and then my dosage would be adjusted and it would start again!
I know it's scary trying new medications, but you can always try it and if you don't like what it's doing to your body you can ask for alternatives.
Goodluck!