Penkywicz, 33, formerly of Leigh Centurions, underwent an out-of-competition test last December which proved positive.
He had previously served a two-year drugs ban in 2005 while with Huddersfield.
UKAD cited the offence as "presence of a prohibited substance or its metabolites or markers in an athlete's sample" and back-dated the ban to April 9.
Penkywicz played his last international against England in 2008.
He has not played this season because of injury.
(Reporting by Steve Tongue; Editing by Julian Linden)
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