If you can't eat whole grains and veggies for some reason, then a dietary fiber supplement may be indicated. Plain water is considered the most healthy of these, because it doesn't have caffeine, sugar, artificial flavors, or other chemicals. First, you need to have a relationship with a health care team that understands MS and will help you care for the functional problems you are experiencing. Second, there are some things you should avoid. Regular laxatives are your worst enemy.
Laxatives also cause cramping and can cause electrolyte imbalances if used chronically.
A second thing to avoid is enemas or colonic irrigations. They also cause the bowel to slow down, and will also cause electrolyte imbalance if used regularly. If, after going on a high-fiber diet, adjusting your drinking habits, and developing a very regular bathroom routine, you're still having problems, it may be time to add one or more medications. Because MS is a multi-system disease, many medications will have unusual side effects in MS patients. Recently, selected patients have had some good results with cisapride, a medication that improves stomach emtpying and gut motility.