Glucosamine and chondroitin are generally safe for most adults. Out of 107 studies reviewed in a large systematic review, 80 reported either no adverse effects, only mild ones, or side effects no different from a placebo. Studies tracking users for three to six years have not revealed major safety concerns at standard doses. That said, there are specific situations and lesser-known risks worth understanding before you start taking these supplements.
Common Side Effects
The most frequently reported problems are digestive: nausea, diarrhea, constipation, upset stomach, and bloating. These tend to be mild and often resolve on their own. Taking the supplement with food typically helps. Beyond gut symptoms, a small number of studies have noted allergic reactions such as skin rashes. Serious side effects are rare in the clinical literature, but they do exist, which is why the sections below matter.
Blood Sugar and Diabetes
For years, there was concern that glucosamine could raise blood sugar, since it plays a role in how the body processes glucose. Clinical trials have not supported that fear. In a 90-day trial of non-diabetic osteoarthritis patients taking 1,500 mg of glucosamine sulfate daily, fasting blood sugar, glucose tolerance, and insulin resistance all remained unchanged. Other studies in people with diabetes reached the same conclusion: at standard doses, glucosamine does not meaningfully affect blood sugar control. If you have diabetes, there’s no strong reason to avoid glucosamine on metabolic grounds alone, though monitoring your levels when starting any new supplement is reasonable.
Liver Safety
Liver injury from glucosamine or chondroitin is rare, but it has been documented. Several case reports describe elevated liver enzymes, jaundice, and in one extreme case, fulminant liver failure leading to death after four weeks of use. Most reported cases involved people who already had underlying liver conditions, such as chronic hepatitis C. In those patients, liver enzymes climbed to four to seven times above normal during glucosamine use and returned to baseline after stopping.
Two additional cases linked a combination supplement containing both glucosamine and chondroitin to liver enzyme elevations, one accompanied by diarrhea and the other showing only mild changes without symptoms. These are isolated reports among millions of users, but they suggest that people with existing liver disease should be cautious and have their liver function monitored.
Eye Pressure and Glaucoma Risk
One risk that flies under the radar is the potential effect on eye pressure. A large study using data from the UK Biobank found that glucosamine users had statistically higher eye pressure compared to non-users, even among people without a glaucoma diagnosis. Data from Finland’s national health registry showed an even more striking signal: people who used glucosamine before being diagnosed had roughly double the risk of developing open-angle glaucoma (hazard ratio of 2.35) and nearly double the risk of glaucoma overall (hazard ratio of 1.95).
This doesn’t prove glucosamine causes glaucoma, but the association is strong enough to pay attention to. If you have a family history of glaucoma or already have elevated eye pressure, it’s worth discussing glucosamine use with your eye doctor.
Interaction With Blood Thinners
If you take warfarin (Coumadin), glucosamine deserves extra caution. The FDA’s MedWatch database contains 20 reports of glucosamine or glucosamine-chondroitin use alongside warfarin leading to increased INR, a measure of how long your blood takes to clot. The World Health Organization’s adverse drug reactions database logged 21 similar reports, and in 17 of those cases, clotting returned to normal after stopping glucosamine.
The mechanism isn’t fully understood, but the pattern is consistent enough that anyone on warfarin or similar blood thinners should let their prescriber know before adding glucosamine. More frequent INR monitoring may be needed, at least initially.
Shellfish Allergies
Most glucosamine is derived from shrimp, crab, or lobster shells, which understandably concerns people with shellfish allergies. The good news: shellfish allergies are triggered by proteins in the meat, not the shells. In a clinical challenge study, every shrimp-allergic participant tolerated 1,500 mg of shrimp-derived glucosamine without any allergic reaction, whether immediate or delayed. The supplements tested simply didn’t contain clinically relevant levels of shrimp allergen.
If you prefer to eliminate even the theoretical risk, glucosamine made from fermented corn (sometimes labeled as vegan or shellfish-free) is widely available and contains no animal-derived ingredients at all.
Pregnancy and Breastfeeding
There is essentially no clinical data on glucosamine use during pregnancy. For breastfeeding, the National Library of Medicine’s LactMed database notes that no studies exist but considers it unlikely to harm a nursing infant. The lack of data isn’t the same as evidence of harm, but it does mean there’s no established safety profile for these populations.
Long-Term Use
Several studies have followed glucosamine and chondroitin users for three to six years. A three-year placebo-controlled trial of glucosamine sulfate in knee osteoarthritis patients and a six-year follow-up study from the Osteoarthritis Initiative both tracked safety outcomes over extended periods. The systematic review encompassing these and other long-term studies concluded that the side effect profile remains largely the same over time: mostly digestive, mostly mild.
A small number of long-term studies flagged possible associations with cardiovascular events or cancer, but other studies of similar duration found no such increase. The evidence on these serious outcomes is mixed and not strong enough to draw firm conclusions either way.
Quality and Regulation
Glucosamine and chondroitin are sold as dietary supplements, not medications. In the United States, that means they don’t go through the FDA’s pre-market approval process for safety and effectiveness. The actual content of a supplement can vary between brands and even between batches. Regulatory warnings do surface periodically. In 2025, the Philippines’ FDA issued a public health warning against a specific Kirkland Signature glucosamine chondroitin product sold without proper registration, citing inability to verify its quality or safety.
Choosing products from manufacturers that use third-party testing (look for USP, NSF, or ConsumerLab seals) gives you a better chance of getting what the label promises, at the dose it claims, without undisclosed contaminants.