CIA doctors considered using a “truth serum” on suspected terrorists in detention after waterboarding appeared to be ineffective and traumatic for US personnel who witnessed it, according to newly declassified documents.

The proposal to use the drug in a programme codenamed “Project Medication” was revealed in a 90-page report by a senior CIA medical officer, which was released on a judge’s order to the American Civil Liberties Union (ACLU), after a prolonged legal battle. The ACLU published the report by the unnamed officer on Tuesday.

The idea of using drugs on US captives in the “war on terror” was recommended as “probably worth a try” by the CIA’s office of medical services but dropped after the agency’s counter-terrorism centre decided not to ask the Department of Justice in George W Bush’s administration for a legal ruling. The department had previously provided legal memos to justify the use of torture like waterboarding and confinement in small boxes.

The CIA report, which reviews the medical office’s participation in detainee operations from 2002 to 2007, shows that the agency’s medical staff played a key role in interrogations in the days after the 9/11 attacks.

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They took part in over 120 rendition flights, taking prisoners to secret CIA detention centres. They helped keep inmates alive and provided the appearance of medical rectitude. The CIA report notes that its doctors were “indispensable” to the effort of “legitimising the programme”.

In the case of Khaled Sheikh Mohammed, the al-Qaida mastermind behind the 9/11 attacks who was waterboarded 140 times, the CIA medical officer said that the torture, which simulates the experience of drowning, “provided periodic relief from his standing sleep deprivation”.

In the case of case of another senior al-Qaida member, a Saudi-born Palestinian known as Abu Zubaydah, his confinement in boxes as small as 30in x 21in x 30in was deemed to “serve as an escape from more severe measures”.

“Without the doctors’ participation none of this would have happened,” said Dror Ladin, an attorney on the ACLU’s national security project. “They were essential participants and completely complicit.”

According to the CIA medical report, a critical document paving the way for the use of waterboarding was a memo from Major Jerald Ogrisseg, the chief of psychological services at the US air force school for survival, evasion, resistance and escape (Sere), which taught pilots and crew to withstand torture if captured.

“The plan is to rapidly overwhelm the subject, while still allowing him the option to choose to cooperate at any stage as the pressure is being ratcheted up,” Ogrisseg wrote in the memo, entitled: “Psychological terms employed in the statutory prohibition on torture”.

“The plan hinges on the use of an absolutely convincing technique,” the memo says. “The water board meets this need. Without the water board, the remaining pressures would constitute a 50% solution and their effectiveness would dissipate progressively over time, as the subject figures out that he will not be physically beaten and as he adapts to cramped confinement.”

The justice department issued three memos which ruled that confinement boxes and waterboarding did not constitute torture. Waterboarding, one memo argued, was “simply a controlled acute episode lacking the connotation of a protracted period of time generally given to suffering”.

Abu Zubaydah, captured and wounded in Faisalabad in March 2002, was the first target of the extreme interrogation measures authorised by the Bush administration. The camp where he was tortured is not identified in the report but was later revealed to be “Detention Site Green” at a Thai military base. The site was run by the current CIA director, Gina Haspel, though she arrived after the worst period of Abu Zubaydah’s torture was over.

It was decided that CIA doctors would not provide hands-on medical care so that the detainee was “given the impression that he could not escape into an alleged need for medical care”. He was given bandages and antiseptic and left to dress his own gunshot wounds.

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On his first day of interrogation, on 4 August, Abu Zubaydah was slapped, slammed into a wall, confined in progressively smaller boxes, and finally waterboarded. It was assumed that the treatment would break his resistance within a couple of days but the CIA medical officer observed he “seems amazingly resistant to the waterboard” and was “becoming habituated to the boxes”.

“Contrary to expectations, the process was going to take ‘a long time’,” the report said. On 8 August, “a particularly aggressive session left [Abu Zubaydah] highly distraught and some of the on-site staff profoundly affected”. The US personnel at the CIA black site decided they would not continue until the leadership at the counter-terrorism centre (CTC) back in the US witnessed the process first-hand. A video conference was set up but they were ordered to continue. The role of the agency medical staff would henceforward include staff counselling.

The Abu Zubaydah interrogation led to discussion of a medical “disinhibitor” and led to a CIA review of the use of “truth serums” which looked at past US, Russian and Chinese experiments. At the end of the 2003 review, informally called Project Medication, the preferred drug recommended was a benzodiazepine drug called midazolam, also known as Versed.

Under international law, there is a prohibition against medical experimentation on prisoners and a ban on the use of “mind-altering drugs”. The CIA lawyers suggested that the legal status of “truth serums” was unclear, but the question became moot as the legal department “did not want to raise another issue with the Department of Justice”.

The CIA report also noted another downside of using drugs. Under its interrogation programmes, harsh techniques were stepped up if it was found that a detainee had provided false information.

“A detainee speaking under the influence of drugs, however, could credibly claim ignorance of anything he had said,” the report said, adding that the failure to pursue drug-assisted interrogation had spared CIA doctors “some significant concerns”.

• This article was amended on 14 November 2018. An earlier version said that at “the end of the 2003 review, the preferred drug recommended was a recently developed drug called benzodiazepine also known as Versed.” The drug known as Versed was developed in the 1970s, and while belonging to the benzodiazepine class of drugs, it is called midazolam. This has been corrected.