Glucosamine is a natural compound found in cartilage — the tough tissue that cushions joints.

In supplement form, glucosamine is harvested from shells of shellfish or made in a lab. There are several forms of glucosamine, including glucosamine sulfate, glucosamine hydrochloride and N-acetyl glucosamine. These supplements aren't considered interchangeable.

People use glucosamine sulfate orally to treat a painful condition caused by the inflammation, breakdown and eventual loss of cartilage (osteoarthritis).

Research on glucosamine use for specific conditions shows:

• Osteoarthritis. Oral use of glucosamine sulfate might provide some pain relief for people with osteoarthritis of the knee. Some research shows that it may also help slow knee joint degeneration associated with osteoarthritis. More studies are needed to determine the benefits of glucosamine sulfate supplements for osteoarthritis of the hip, spine or hand.
• Rheumatoid arthritis. Early research suggests that oral use of glucosamine hydrochloride might reduce pain related to rheumatoid arthritis. However, researchers didn't see an improvement in inflammation or the number of painful or swollen joints.

When considering glucosamine, read product labels carefully to make sure you choose the correct form. There's less clinical evidence to support the use of N-acetyl glucosamine in treating osteoarthritis, and more research is needed to confirm its benefits.

When taken in appropriate amounts, glucosamine sulfate appears to be safe. Oral use of glucosamine sulfate can cause:

• Nausea
• Heartburn
• Diarrhea
• Constipation

Other side effects may include:

• Drowsiness
• Skin reactions
• Headache

Because glucosamine products might be derived from the shells of shellfish, there is concern that the supplement could cause an allergic reaction in people with shellfish allergies.

Glucosamine might worsen asthma.

There's some concern that glucosamine might raise eye pressure. If you have glaucoma, talk to your doctor before taking glucosamine supplements.

Possible interactions include:

• Acetaminophen (Tylenol, others). Taking glucosamine sulfate and acetaminophen together might reduce the effectiveness of both the supplement and medication.
• Warfarin (Jantoven). Taking glucosamine alone or in combination with the supplement chondroitin might increase the effects of the anticoagulant warfarin. This can increase your risk of bleeding.