Collins addressed the 16 chief executives at a meeting on Wednesday before firing off his email on Thursday afternoon.
"Players should be particularly aware that they are subject to testing in the period immediately following their team's elimination from the NRL competition," Collins wrote.
"End-of-season celebrations and the like are a risk period for players and their clubs, so please take this opportunity to remind your playing group about the NRL's expectations and rules in relation to illicit drugs."
Under the heading of behaviour, he went on to warn the CEOs of the NRL's stance on any player stepping out of line.
"Can I also ask that you ensure that your club has measures in place to reduce the risk of unacceptable behaviour during any end-of-season events, and that your players and staff understand the importance of adhering to the high standards of personal behaviour at these events and during the off-season," he said.
Most clubs will hire professional security guards to escort their players to venues.
One group of players however are taking a risk by booking a private room at The Star for their celebrations.
Mad Monday is the traditional days for "nudie runs" where players who haven't scored a try have to streak in front of the rest of the team.
Bulldogs boss Raelene Castle is a supporter of Mad Monday.
"It's like the office Christmas party," she said. "The players and the staff are entitled to go out for good time.
"We've got a plan in place - there's a full briefing for the boys, full security, and rules and expectations.
"The players understand their responsibilities as high profile athletes."
