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You are right. GNC Vitamin supplements (especially the mega , ultra mega ones) do contain high doses of certain vitamins. Eventhough such high doses are never recommended it is unlikely that you will develop side effect from this, especially anxiety or mood related effects. However since there was no precipitating factor for anxiety I would entertain this possibility.
Vitamins A, D, E, K and to some extend B12 are stored in the body- mostly in the liver. These might be stored for months. Other B complex Vitamins are water soluble and hence it is easily excreted from the body in few days (less than a week). The good news in that the mega doses in GNCs are of mostly B complex vitamins and hence it will be easily excreted in weeks. B12 supplements in high dose are safe and will not cause anxiety or mood related issues.
Hope this helps
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