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.
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.
In May that year, Nationwide News published a leaked extract from an independent report commissioned by Cronulla into the use of peptides in the club's supplements program in 2011.
The report questioned whether Mannah's cancer relapse could be linked to peptide use, and is at the centre of the defamation action launched by Dank.
He told the court the claims were wrong and were part of a series of articles that caused his once-busy business to flatline and invitations to social functions to dry up.
Under cross examination from Nationwide News' barrister Tom Blackburn SC, Dank said he had advised Mannah he thought peptides CJC 1295 and GHRP 6, which stimulate the release of growth hormones, "were safe to use".
"Growth hormone does not stimulate the proliferation of cancer cells," Dank, who earlier said he had no medical qualifications, told the court.
The jury also heard Dank didn't tell Cronulla team doctor David Givney he was advising players about peptide use or was also organising for players to be to injected.
"That was (trainer) Trent Elkin's responsibility," he said.
Dank also claimed the peptides weren't given to players in doses sufficient to increase their performance or enhance recovery.
"Really, we were looking at changes to sleep performance," he told the three men and one woman on the jury.
The legal action was also launched in response to other articles which asserted police were investigating Dank's activities, which he has said was not true and caused many people to question his ethics in public.
The hearing continues before Justice Lucy McCallum.