Sydney Sports scientist Stephen Dank has told a Sydney court that he instructed an NRL player who later died of cancer that injecting peptides was safe.

He also said he was administering the supplements to improve players' sleep, not their performances.

Sports scientist Stephen Dank. Credit:Justin McManus

Dank made the comments on Tuesday at the NSW Supreme Court where he is suing Nationwide News, publisher of Sydney tabloid The Daily Telegraph, over a 2013 front-page article which questioned whether Cronulla Sharks NRL player Jon Mannah's death was accelerated by the use of peptides.

Mannah died from Hodgkin's lymphoma in January 2013, aged 23.