When you hear about all of the benefits of vitamin C, especially for boosting the immune system, it makes you want to take as much as possible. With vitamin C, I'm not too concerned about short term usage because it is so widely viewed as safe even at fairly high doses. But some ME/CFS "gurus" recommend enormous amounts of vitamin C...up to 10,000mg per day! It seems to me, this can't be safe for long term usage. So, in this post, I'll try to determine the maximum safe dosage that can be take for extended periods of time--i.e. years at a time.
In the U.S., the recommended daily allowance (RDA) of vitamin C is 65mg/day for women and 75 mg for men. (citation). Most multivitamins contain anywhere from 5 to 10 times the RDA or more, so exceeding the RDA by even large amounts doesn't seem to be harmful.
But, I've seen at least one ME protocol that called for 4,000 mg a day, every day, and one that called for 10,000, for a limited period of time.
Here's a collection of quotes from different medical sites about the maximum safe doses for vitamin C:
"Some studies suggest that adults should take 250 - 500 mg twice a day for any benefit. Talk to your doctor before taking more than 1,000 mg of vitamin C on a daily basis." (Univ. of Mayland Med. Cntr.)
"At least one study has suggested that very large vitamin C doses can cause rather than prevent oxidative damage to DNA in cells. The [US National Academy of Sciences] panel set a 2,000 milligram daily upper limit for vitamin C from a combination of food and supplements." (BBC article)
"People can take up to about 1000mg safely." (VitaminsDiary)
"In 2000, the Food and Nutrition Board of the Institute of Medicine set a tolerable upper intake level for vitamin C at 2,000 mg per day. Taking more than 2,000 mg per day is likely to increase the risk of side effects." (Livestrong)I could post more examples, but you'll have to take my word that the numbers 1,000 and 2,000 came up over and over.
My conclusion is to limit my intake to about 1,000 mg/day (1,333% of the RDA for men) on a normal day, and increase it to 1,500 only when I feel I may be getting sick....or, rather, more sick.