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Side Effects of N-Acetylglucosamine

by 
author image Dr. Tina M. St. John
Tina M. St. John runs a health communications and consulting firm. She is also an author and editor, and was formerly a senior medical officer with the U.S. Centers for Disease Control and Prevention. St. John holds an M.D. from Emory University School of Medicine.
Side Effects of N-Acetylglucosamine
N-Acetyl glucosamine is one of the forms of glucosamine. Photo Credit: Farion_O/iStock/GettyImages

N-acetylglucosamine (NAG) is an amino sugar, a derivative of the simple sugar glucose. Your body produces NAG, which serves as a building block for many biochemicals that serve wide array of diverse functions.

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High concentrations of NAG-containing biochemicals are present in your skin, bones, joints, tendons, eyes, blood vessels, heart valves, liver, lungs, and nervous system. NAG as well as the closely related chemicals glucosami

ne hydrochloride and glucosamine sulfate are available as over-the-counter nutritional supplements. NAG and other forms of glucosamine are generally considered safe with uncommon serious side effects.

Minor Side Effects

In a hallmark study reported in the journal Pharmatherapeutica involving more than 1,200 people taking glucosamine supplements for joint pain, side effects were uncommon and usually minor. These included:

  • Mild to moderate stomach pain
  • Stomach upset
  • Heartburn
  • Diarrhea
  • Nausea
  • Constipation
  • Drowsiness

Increased Eye Pressure

Increased internal eye pressure, also known as ocular hypertension, can progress to vision-threatening glaucoma. A small study published in 2013 in JAMA Ophthalmology reported increased internal eye pressure associated with taking a glucosamine supplement.

A larger study published in 2017 in the journal Eye reported that people taking a glucosamine supplement were more likely to experience a significant increase in internal eye pressure over a 3-month period compared to those not taking the supplement. This effect was most pronounced in older study participants.

Allergic Reaction

Many glucosamine supplements are produced from the shells of shrimp, crab and lobsters. This raises a theoretical concern about the potential for an allergic reaction to glucosamine supplements among people with a shellfish allergy. However, the proteins that trigger an allergic reaction are found in the meat of shellfish not the shells.

A small study conducted among people with a shrimp allergy found none had a reaction to glucosamine supplements produced from shrimp shells, as reported in 2006 in the journal Clinical and Experimental Allergy. Another small study found no allergic reactions to glucosamine supplements among people with an allergy to crab, shrimp and/or lobster, as reported in 2004 in the Journal of Allergy and Clinical Immunology.

These studies offer reassurance about the safety about glucosamine supplements in people with a shellfish allergy but there has been a single report out of Spain of an allergic reaction, which was reported in the journal Allergy in 1999. Given that 6.5 million adults in the United States take some form of glucosamine (with or without chondroitin), according to the National Center for Complementary and Integrative Health, it appears that the risk of an allergic reaction is extremely low, even among people with a shellfish allergy.

Worsening Asthma

A single case of worsening asthma was reported in 2002 in the Journal of the American Board of Family Practice. A 52-year-old woman with previously well-controlled asthma experienced worsening symptoms when she began taking a glucosamine-chondroitin supplement.

Her symptoms improved after she stopped taking the supplement. Although there have been no additional similar reports, people with asthma should watch for increased or worsening symptoms when starting NAG or another type of glucosamine supplement.

Other Considerations and Precautions

Although they are very closely related, NAG, glucosamine hydrochloride and glucosamine sulfate may not necessarily have the same effects in the body. To be sure it's safe for you to take a glucosamine supplement and to help you determine the best form for the desired effect, talk with your healthcare provider beforehand.

This is especially important if you take a blood thinner or have a shellfish allergy, asthma, ocular hypertension, glaucoma or risk factors for glaucoma, such as diabetes or high blood pressure.

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