Hi, I'm a university student who has crohn's, and I'm actually doing a presentation on Crohn's in a few more days. I came across some literature that mentioned taking 1-2mg of folic acid daily can prevent side effects such as stomatitis and nausea. While methotrexate is a folate antimetabolite, it is known to possess a variety of other immune modulating and anti-inflammatory mechanisms. These include inhibiting interleukins (IL-1, IL-2, IL-6, IL-8), which are key signalling molecules in immune molecular pathways. Also, methotrexate causes induction of adenosine, which alters even more biological processes. Moreover, methotrexate in Crohn's seems to still work when folic acid is administered, which indicates these other mechanisms play a role.
In my personal experience reading these forums, I have noticed that most people start with just 1mg of folic acid daily, then slowly bump up their dose, one milligram at a time until side effects are minimal. I haven't heard of many people taking more than 5mg of folic acid daily so I would check with your doctor about such higher doses.
Overall, from what I read about the efficacy of this drug, it appears methotrexate is good for inducing remission and maintaining remission.