The theme park chain SeaWorld, already facing wide criticism from animal rights advocates, treats some of its marine mammals with psychoactive drugs, according to a document obtained by BuzzFeed.

Trainers give their orcas, also known as killer whales, the psychoactive drug benzodiazepine, according to the sworn affidavit filed in the Ontario Superior Court of Justice in a dispute between the park company and the rival company Marineland over the transport of a prized killer whale, Ikaika, to SeaWorld.

Benzodiazepines are a type of drug that includes the common human medications Valium and Xanax. The orcas' mental health issues, SeaWorld's critics say, are a direct result of their keeping the mammals in captivity.

Jared Goodman, Director of Animal Law at the People for the Ethical Treatment of Animals (PETA), told BuzzFeed that he believes the leaked documents will play a key role in SeaWorld's future.

"The veterinary records show that orcas at SeaWorld are given psychotropic drugs to stop them from acting aggressively towards each other in the stressful, frustrating conditions in which they're confined instead of funding the development of coastal sanctuaries - the only humane solution," Goodman said.

The questions about the drugs given to the whales, which also include a range of antibiotics, come as SeaWorld is reeling from a critical documentary. Blackfish tells the story of a killer whale named Tilikum, who's been accused of killing three people but is still retained by SeaWorld. Tilikum's genes are found in 54% of the whales in SeaWorld's current whale collection, and has fathered at least 21 whales from artificial insemination.

A spokesperson for SeaWorld Fred Jacobs defended the medication in an emailed statement.

"Benzodiazepines are sometimes used in veterinary medicine for the care and treatment of animals, both domestic and in a zoological setting," Jacobs said. "These medications can be used for sedation for medical procedures, premedication prior to general anesthesia, and for the control of seizures. The use of benzodiazepines is regulated, and these medications are only prescribed to animals by a veterinarian. Their use for cetacean healthcare, including killer whales, is limited, infrequent, and only as clinically indicated based on the assessment of the attending veterinarian. There is no higher priority for SeaWorld than the health and well-being of the animals in its care."

But animal advocates say the orcas' condition is far from normal.

The founder of the Orca Research Trust, Ingrid Visser, said the drugs are likely treating a condition caused by captivity, and that their violence is the result of stress, not native aggression.

"They do not cope with being kept in these tanks. They survive to some degree, but they don't thrive to any degree," Visser said. "They show stereotypical behaviors that are abnormal, repetitive behaviors like head bobbing, chewing on concrete, and self mutilation by banging the side of their heads on the side of the tank, and there isn't a single orca living in captivity where you cannot see one of these behaviors, and in many of them you see multiple examples of these behaviors."

PETA's president, Ingrid Newkirk, accused SeaWorld of "pump[ing] these marine slaves full of psychotropic drugs in order to force them to perform stupid tricks."