I've been on 6MP for around 20 years now and haven't had any noticeable side effects. It's very similar to Imuran. All medications have possible side effects unfortunately. Untreated Crohn's can bring a lot of problems as well that are just as scary. Remember that if you do experience any side effects you can stop the medication right away before anything gets out of hand. Medications like Imuran, 6MP and Methotrexate require constant monitoring with regular blood work. As long as you keep up with the blood work you'll reduce your risk of serious irreversible side effects.