Glucosamine is a natural substance made in the body from glucose. It is believed to promote the manufacture of proteins that help build and maintain cartilage. Loss of cartilage is a common cause of knee pain. Most glucosamine supplements are made from crab, lobster and shrimp shells. Glucosamine may interact with certain diuretics and cause people to need higher doses of the diuretic. People taking diuretics may be more likely to experience gastrointestinal discomfort when taking glucosamine.