Green’s Dictionary of Slang

(drugs) an amphetamine.

[1926		Discobolus (Columbia U. N.Y.) 3:2 2: Unfortunately many quacks, charlatans, and ‘physical culture’ exponents have also been attracted to the field, and the results have been a flooding of the market with ‘sure cures’, ‘easy to take fat reducing tablets’ [and] ‘pep pills’].
1937		Time 10 May 45/1: [heading] Pep-pill poisoning. [...] Students who, while cramming for final examinations, collapse, [...] are under suspicion of using the substance. They call it ‘pepper-up’, ‘pep pills’.
1949		Eve. Teleg. (Dundee) 18 Mar. 8/2: Latest sportsmen to use ‘pep’ pills are the Earls Court Rangers ice hockey team.
1959		A. Sinclair My Friend Judas (1963) 86: Twenty times the number of peps and tranquilizers to keep them happy.
1961		T. Taylor Baron’s Court All Change (2011) 99: They were hitch-hiking [...] pep tablets and trad-dogs, with a uniform of dirty jeans.
1966		G. Fletcher Down Among the Meths Men 46: Several were takling pep pills, the so-called purple hearts.
1969		Cressey & Ward Delinquency, Crime, and Social Process 817: The widespread use of pep-pills and methedrine crystal.
1971		M. Novotny Kings Road 77: They offered an assortment of pep pills and grass, occasionally acid.
1981		G. Young Slow Boats to China (1983) 165: Abdullah [...] began furiously stuffing handfuls of qat leaf, the Yemeni equivalent of pep pills.
1997		(con. 1960s) A. Frewin London Blues 85: The police are having a big crackdown on pep pills.
2001		ONDCP Street Terms 16: Pep pills — Amphetamine.