I haven't tried this med, but I went online to look and it said this was a common side effect. You might need to see a gastroenterologist if you haven't already to treat what is going on with your digestive system. It might just need a simple med and some time to clear it up. I hope you get this solved soon. I know how miserable this can be.

I was on this drug for a few years. I had daily, sometimes uncontrollable diarrhea and did see a gastroenterologist to rule out other issues. He took me off the sulfasalazine that I was also on. The diarrhea continued. My rheumatologist put me on Humira and then took me off the leflunomide. The diarrhea eventually went away, but it took awhile after I was off of it. Apparently it stays in your system awhile. I was on 20 mg's too. Like you, I thought the drug was helping so stuck it out for a while, but it had gotten to the point where I was afraid to leave the house, and never without taking Imodium.

Thankfully my cultures came back negative for bacteria and after having been off the Leflunomide for 9 days I fell better. I just started taking the 10 mg dose with some diarrhea returning but so far not as bad as before. I think it may be manageable as long as it doesn't get worse but I will update the rheumatologist to let him know. I was on Humira too for about 5 years along with MTX until last fall when I was taken off the Humira due to a breast cancer diagnosis. I didn't think that it was helping that much anymore but at least I didn't have problems with any side effects from that drug. I am hoping that I can tolerate this drug because it seems to be helping mostly. I had carpal tunnel pain and some fingers were tingling/numb for the first time after being on the 10 mg dosage and then remembered the doctor warning me that in some cases you can get neuropathy from the medication.
Did you have to take anything to remove the Leflunomide from your body before changing medication? I read that in certain cases when the doctor needs to take someone off the medication that there something given to wash it out of the body. It stays in the system for a long time and I will need to find out more about that from my doctor. Reading that bothers me.

I have been on this medication for a couple years and have not had this side effect.

I have been off this drug now for 3 weeks and still have the same problem side effect but not as frequent. I have been to a GI doc and he agreed that it must be the Leflunomide causing the diarrhea and that I am at the half life point of the drug and that if frequency is still a problem in a month then it may be time for a colonoscopy. I am hoping to avoid that and that is just a case that it needs to clear out of my body like poodlemom experienced. Besides diarrhea I also have had an increase in blood pressure so my BP drug dose has tripled. I can't say for sure if this is due to Leflunomide but that is another side effect of this drug. I am feeling better RA wise but won't go back on this medication again. My next drug will be an immunosuppressant called Azathioprine. I am hopeful that I don't experience any side effects this time. Thanks for all your responses.

I was in Arava for 2 years. I never had this side effect, but it helped spike my liver enzymes and didn't work well, so I went off of it.