Because salmon is considered to be a very oily fish, it can sometimes cause problems with diarrhea. Those who regularly take fish oil supplements might find themselves running to the bathroom more than they would like, simply because this is one of the most common side-effects. The same kind of thing can happen with those who eat salmon on a regular basis.
If you like to eat salmon a lot, you might just find that you will have problems with diarrhea once in a while, especially if you eat a lot of it. Although it is true that salmon can cause some gastrointestinal problems, it is usually nothing more serious than gas or a mild case of diarrhea.