Russian authorities allege drugs were found on the Greenpeace protest ship which two New Zealanders were arrested on last month.
The environmental group has ridiculed the claim, saying morphine was kept legally in a safe on the Arctic Sunrise for medical purposes.
The ship's crew of 28 activists, including New Zealanders Jonathan Beauchamp and David Haussmann, plus two freelance journalists, were arrested on September 18 by Russian special forces in international waters during a protest over Arctic oil drilling.
They are being held in a Russian prison while they face piracy charges which could see them jailed for up to 15 years if convicted.
Russian authorities have revealed they are considering laying further charges against the crew after reportedly finding morphine and poppy straw, an opiate, aboard the Arctic Sunrise.