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Do Vitamin C Supplements Go Bad or Lose Potency?

author image Adam Cloe Ph.D./M.D.
Adam Cloe has been published in various scientific journals, including the "Journal of Biochemistry." He is currently a pathology resident at the University of Chicago. Cloe holds a Bachelor of Arts in biochemistry from Boston University, a M.D. from the University of Chicago and a Ph.D. in pathology from the University of Chicago.
Do Vitamin C Supplements Go Bad or Lose Potency?
Vitamin C tablets degrade over time. Photo Credit: brozova/iStock/Getty Images

Vitamins supplements are commonly taken by many people to treat a variety of health problems. Over time, however, vitamins can degrade, causing them to become chemically altered. Although vitamin C degrades, it is not toxic. The speed with which vitamin C degrades depends on a number of factors, and even expired vitamin C is "safe," though it may not be effective.

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How Does Vitamin C Degrade?

Vitamin C is also known as ascorbic acid or ascorbate. It can undergo a type of chemical reaction known as oxidation, which changes its chemical form. The oxidized product of vitamin C is known as dehydroascorbic acid, or DHAA. Although this compound can be converted back into vitamin C, that chemical reaction does not occur naturally.

Product Quality

Although DHAA isn't toxic, it does not function effectively as vitamin C, meaning that over time, vitamin C supplements become less potent. However, different vitamin C supplements turn into DHAA at different rates. One factor that controls this is the initial quality of the supplement. Supplements that are packaged with very little moisture and are mostly free of any oxidized compounds will break down more slowly than poor quality supplements. However, there are no large-scale studies that have tested the degradation of different kinds of vitamin C, so it is not known which brands are the "best" in terms of expiration date.

Storage Conditions

Another thing that affects how quickly a vitamin C supplement is broken down is how it is stored. Once the container is open, the supplement will begin to degrade more quickly, so unopened vitamin C supplements have a longer shelf-life than opened ones. Keeping vitamins in the bathroom or kitchen, where the humidity can rise, can increase the rate at which they degrade. Vitamins should be stored in a cool and dry part of the house, such as the bedroom.


Not all vitamin C supplements have printed expiration dates, and you can still take old vitamin C tablets. However, if they have changed color or begun to dissolve, they won't be as effective. Do not take higher doses of the supplement to counteract this loss of potency, however, because you may accidentally take too much vitamin C. This can cause diarrhea and other health problems.

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