Constipation is one of those things that can have a real impact on your quality of life because it makes you feel like… well, er… shit, but I bet most non disabled people don't even consider how disability can affect such a basic, important function.
The others have listed some of the causes like inactivity, low fluid intake, (many people with MS restrict this to avoid having to run to the toilet every few minutes), not enough fibre etc., Some of which you might be able to do something about, some not.
There are different forms of constipation, and laxatives work in several different ways to deal with them, (grisly details coming up):
If you have plenty of push but the damn thing feels dry and hard, and it's like trying to force a pineapple backwards up a hosepipe, you may benefit from a bulk forming laxative like Metamucil which makes the stool softer and easier to pass or a lubricant like some suppositories or mineral oil. If the stool is OK, but you lack the urge or the ability to push, (a nurse once said to me, " after all, your insides have got MS too"), you may need a stimulant such as Senna. Saline laxatives like Milk Of Magnesia help to draw water into the colon and thus soften the stool, and may be mildly stimulatory.
Don't forget that some laxatives can interact with some medications or prevent them from being absorbed, so you may want to try a different tack. There is a small machine available which provides a colonic massage – it sits on your lap with a strap running round your back, and two arms gently manipulate your stomach area. With daily use it is apparently very effective and my sister, who is a nurse, says that in the old days they used to do this massage manually with great success.
Jimmylegs, am I right in thinking you found magnesium helped with your swallowing difficulty? If so, it might help with both problems for Tiramisu,