I was on azathioprine for 4 weeks, starting in October at 25mg/day and increasing to 50mg/day after the first week, then 100mg/day the second week before a final increase to 150mg/day.
I had weekly blood tests to check for any side effects. After a few weeks they noticed my blood platelet level had dropped, but was asked to carry on in the hope that my body would recover. After another blood test, my platelets had dropped to a very low level (12) so was advised to stop taking the azathioprine.
After a further month (off the azathioprine) I developed two big, ugly, black bruises on the back of my right thigh, so I had another blood test that showed that my platelet count had dropped further to a dangerous level (3!)
So I had to go back to hospital and have a bone marrow biopsy to see if my body was either not producing enough platelets or making a normal amount in the marrow - but being destroyed while in my bloodstream. Fortunately, it turned out to be the latter, so they just gave me a(nother) dose of Prednisolone. Turns out that my over-active immune system is wiping the platelets out so that they can't build up again...
As a side note, the bone marrow biopsy was the most horrific procedure I've ever had to endure. Even though I had a local anaesthetic applied, the pain was worse than anything I've ever experienced in my life. It's been three days since having it done and my back still aches and moving around is still painful. If I'm ever asked to have it done again, then they are going to have to knock me out beforehand!