Considering there is no data showing any long term effects I'd say that says a lot. GW is extremely safe. Probably one of the safest things you can run. As a matter of fact in many ways it improves health, because it lowers blood pressure, reduces heart rate and improves cholesterol. It also gives stable blood sugar levels.
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there are no studies that have concluded with any long term negative effect except one that was very flawed and refuted a million times over... if your asking for something showing an excess of 10 years, it has not even been around that long so no study can conclude that but as of now, there is nothing showing any sort of problem