IIRC his friends asked him to stop for a month, and then to see how he felt. "You have put back mathematics by a month" he said at the end, and promptly went back on the pills.

Yes, I saw that too. But amphetamines have some nasty side effects.

Methamphetamine was the standard military "go pill" for decades. Then dextroamphetamine. Now there's interest in modafinil, which has milder side effects.[0,1]

[0] http://www.ncbi.nlm.nih.gov/pubmed/22764609

[1] http://goflightmedicine.com/stimulants-sleep-aids/