Daily use of high-dose vitamin C supplements may increase the risk of cataracts. An eight-year Swedish study of 25,593 women showed a 25% increase in cataract removals among those taking vitamin C supplementation (approximately 1,000 mg per day) compared to those who did not. A smaller, but non-significant increase was seen among those taking a multivitamin. Women taking vitamin C supplements for at least 10 years prior to the study had even higher increases in cataract removals (46% increase), as did those on hormone replacement therapy or corticosteroids.
These studies don't prove causation, they prove possible causation and possible coincidence, since they don't account for all the variables involved. There are tons of these studies around because we can't really take people and experiment on them. So while I'd pay attention to them, I wouldn't base my life choices on them. That being said, coumadin, vitamin c isn't the most important antioxidant for the eyes -- the two most important are glutathione and lutein. Glutathione isn't regarded as very well absorbed in supplement form, but you can take the amino acids that manufacture it in the body. It's also the most important antioxidant for the liver. Ask your doc if perhaps you could reduce the vitamin c if you increased these two, and also ask him about bilberry, a very good good antioxidant specific to the eyes.
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