Some people have primarily constipation, others have diarrhea as the major symptom and still others alternate between the two. Occasionally people complain of nausea in addition to the above symptoms. It appears that people who have the syndrome have an hereditary predisposition to the problem. It tends to run in families. Usually, though, there are certain stimuli that set it off.
Some people are sensitive to certain foods, also, that can cause the problem to act up.
IBS is not a dangerous disease. It causes no harm to your system. It can, however, be quite uncomfortable or even downright painful. The underlying cause appears to be inappropriate tightening of the muscles in the colon. When they contract too slowly or too weakly the stool slows down leading to constipation.