Not sure if I can explain, but I had never heard of this and am intrigued being that I have noticed a similar effect with Zinc supplementation while taking adderall. The study referred to came up on first page of pubmed search for amphetamine+ zinc http://www.ncbi.nlm....les/PMC3037197/
One other thing to consider is that zinc is an aromatase inhibitor, which can have a noticeable effect on cognition. Allowing some aromatization of androgens to estrogens is beneficial for cognition (in my experience and I believe I've read some anecdotal and scientific reference to this). So excessively high doses of zinc would theoretically be detrimental to cognition due to the excessive reduction in aromatization alone. That is, independent of whatever other effects Zinc may have on amphetamine dosage.
I'm sure its highly individual, but taking 50mg of zinc picolinate (without any copper) is too much for me. Causes mild anxiety (esp in conjunction with adderall at standard dosage I take), lack of verbal fluency, and blunting of creativity and free flow of thinking. Perhaps it is boosting the effect of adderall and causing excessive levels in my system. And perhaps inhibiting aromatase. Productivity drops significantly within a few days on that dose of zinc. On the plus side, my voice becomes significantly deeper within 24 hours every time but its not worth the trade off. I've settled with 15 to 25mg daily. Periodically will switch it up from plain zinc picolinate to opti-l-zinc which contains some copper, ensures I'm not depleting copper too much.
