Metamucil should be used with caution in pregnant women, children, anyone on medication or patients who have a serious condition such as constipation caused by a bowel obstruction or appendicitis, as the Mayo Clinic states. People who are allergic to psyllium should not use Metamucil either, according to Drugs.com.
Metamucil is a bulk-forming laxative that works by absorbing water and prompts the body to have a contraction of the intestinal muscles, as MetaWellness.com describes. If enough water is not taken with Metamucil, bloating, cramping or worsening constipation can occur. The Metamucil product page provides more information about how works in the stomach and small intestine.