1. Resveratrol is a phyoestrogen which is a substance that can mimic the effect of estrogen much in the same way that soy does. We don't know whether women who have breast cancer or who are at risk for breast cancer should avoid resveratrol. More research and more information is needed.
2. Since it can act as an estrogen-like substance we don't know what kind of problems can occur when men take resveratrol in high doses.
3. The purity of many of the current natural resveratrol products is a potential question. Many of the resveratrol products on the market today will probably be made from Japanese Knotweed. Some will be made from the skin of certain grapes which will likely be much more expensive. More research needs to be done to determine if one source of resveratrol is better than the other.
4. No one knows the correct dosage for humans. Most of the research that has been done was done on animals. Some say 30mg a day is appropriate and others say 500mg. Until a human clinical trial can shed some light on the proper dosage for humans, it is all just guesswork. If 30mg is all humans need and people are taking 500mg, that could be a problem. There is an old saying – "The dose makes the poison." It wouldn't be safe to drink 10 gallons of water nor would it be safe to eat 10 pounds of spinach. We need to know more.
5. Some products indicate the substance in the product is resveratrol while others indicate their product contains trans-resveratrol. This represents additional confusion in the marketplace.
6. Many of the resveratrol supplements on the market will likely be extracts and will be made from Japanese knotweed. Some of these products will have a resveratrol purity level of 98% to 99%. Some will have a purity level of as low as 50%. Japanese knotweed also contains a substance called emodin which has been used as a laxative. Emodin can cause diarrhea. Products with lower levels of purity will contain higher levels of emodin which can cause intestinal distress.
7. Additional warnings can be found here
Resveratrol Adverse Effects and Unknowns
Original article can be found at: http://en.wikipedia.org/wiki/Resveratrol
"While the health benefits of resveratrol seem promising, one study has theorized that it may stimulate the growth of human breast cancer cells, possibly because of resveratrol's chemical structure, which is similar to a phytoestrogen. However, other studies have found that resveratrol actually fights breast cancer. Some studies suggest that resveratrol slows the development of blood vessels which supresses tumors, but also slows healing. Citing the evidence that resveratrol is estrogenic some retailers of resveratrol advise that the compound may interfere with oral contraceptives, and that women who are pregnant or intending to become pregnant should not use the product, while others advise that resveratrol should not be taken by children or young adults under eighteen, as no studies have shown how it affects their natural development. A small study found that a single dose of up to 5 g of trans-resveratrol caused no serious adverse effects in healthy volunteers."