Excellent source, but results may vary
Five stars to ND -- I have bought many, many products over the past months. And I can say without hesitation that they are the best around.
Pyritinol is one of the first nootropics that I tried, I believe eight years ago (when I purchased from a vendor who has long since curtailed its offering, and whom I've not gone back to in years). In the past few years, I've bought from other vendors, and the results have been null and, worse, negative. I'd nearly written it off.
Pyritinol gives immediately noticeable effects. ND's Pyritinol gives me the same positive results as I recall from my first experience -- a decided boost in mental clarity. Perhaps this is something to cycle, but the results seem to diminish within a few days, those that you feel, anyway. Each time I've taken it, going years back, I've gotten small rashes on my hands or arms (going back to previous vendors). There is one study that cites a remote risk, I should note, but Pyritinol is generally regarded as safe, and studies go back to its development to the '60s.
Bottom line, I personally Phenylpiracatam a better alternative (and I keep a stock of ND's on hand). However, I would recommend ND's product if you're interested in giving Pyritinol a try.