Two peptide-based supplements given to Sharks players are reportedly of concern to ASADA.
Fairfax has been told players are considering legal action against the club, claiming they were told the substances were legal.
The Sharks are expected to address the matter at a news conference on Thursday.
The club sought to reassure fans with a short statement on its official website.
"Sharks fans and all rugby league supporters can be assured the club has been very proactive in fully co-operating with ASADA and taking other measures that prioritise the integrity of our club and the welfare of our playing group," it said.
As the crisis cast a pall over the NRL season start on Thursday night, bookmakers suspended betting on Sunday's Cronulla-Gold Coast game.
Read more: League to screen as Sky TV, NRL reach last-minute deal
- AAP
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