It is generally recommended that patients receiving anti-TNF therapy also go on MTX or Imuran. The MTX/Imuran provide two benefits - patients are more likely to achieve remission and stay in remission with combo therapy than with anti-TNF alone; additionally, patients are less likely to develop anti-bodies to the anti-TNF agent if they are on MTX/Imuran. The dose is generally small (you'll notice it's probably called "low-dose methotrexate" if you have looked it up), so the side effects are rarer. My GI has also told me that they generally see fewer side effects in Crohn's patients than in RA or other patients taking the medication (this is anecdotal from a hepatologist, I have not seen any papers to verify this).
A paper that I have appreciated on the current approaches to Crohn's treatment is the 2009 Consensus Report from the Canadian Association of Gastroenterology (
www.cag-acg.org/uploads/cag_cpg_crohnsmar2009.pdf). A consensus report is where the specialists get together and vote on certain common approaches to treatment to determine what
should be done in the majority of cases.
If you're interested enough to take a look at the Consensus Report you'll see that
Statement 12 concerns whether patients derive benefit from taking anti-TNF and immunosuppressive (e.g. MTX) therapy at the same time. The votes were 72% agree strongly and 28% agree with minor reservation. The grade was low - essentially meaning there's not enough research available to make a confident decision.
Statement 11 concerns whether you can reduce antidrug antibody formulation by i) Regular therapy, ii) Concomitant immunosuppressive therapy (MTX), iii) Pretreatment with corticosteroids. The votes were 52% agree strongly, 44% agree with minor reservation, 4% agree with major reservation. The grade on this was high - meaning that they are very confident in the decision and it's unlikely to change.
Probably too much information, but I was very interested in this as well (I was recently put on MTX in addition to my Humira) so I have done a lot of research on it.