Glucosamine and methyl sulfonyl methane, or MSM, serve as dietary supplements. You may take them as individual supplements or combine them together. Most commonly used to help ease the joint pain of arthritis, glucosamine and MSM may also offer other health benefits. However, certain health or medical conditions may make these supplements unsafe.

Cartilage is the durable connective tissue that protects your joints. Glucosamine, a molecule synthesized by your body, helps build and maintain the cartilage you need to keep your joints in good working order. Glucosamine is also abundant in the outer shells of crustaceans, which is the source of glucosamine supplements and affects its safety profile. MSM derives from food sources such as meat, milk products, fruits, vegetables and chocolate. Although your body doesn manufacture MSM, nor do you require it for any functional reason, you likely consume MSM in your daily diet, and this fact may affect how safe MSM supplements are.

According to New York University Langone Medical Center, the most common indication for glucosamine and MSM is to relieve the pain caused by osteoarthritis. These supplements may also decrease the ache of rheumatoid arthritis. Other potential uses for glucosamine and MSM, either separately or in combination, include treatment of temporomandibular arthritis, rosacea, interstitial cystitis and irritable bowel disease.

Although glucosamine may be safe for much of the population, you may need to avoid it to prevent serious adverse effects in certain instances. If you are allergic to shellfish, glucosamine may trigger an allergic reaction because it is isolated from the shells of shrimp, crab and lobster. If you take the blood thinner warfarin, glucosamine may increase the effect of the drug and lead to serious bleeding or bruising. Conversely, some anti-cancer drugs may show decreased efficacy in the presence of glucosamine. Because of limited safety data in pregnant or breastfeeding women, the University of Maryland Medical Center recommends avoiding glucosamine while you are pregnant or nursing.

As a substance found naturally in many food sources, MSM is considered safe as a dietary supplement, even in very high doses, according to New York University Langone Medical Center. In a study noted in the October 2002 issue of "Food and Chemical Toxicology," rats ingesting up to 2 g MSM per kg of body weight -- corresponding to 160 g in a person weighing 175 lb. -- every day for 3 months showed no negative effects at all.