The doctor said military officials searched the cells after two prisoners overdosed on prescription drugs in May and found stashes of pills.
Pointing to a box of prosthetic limbs, the doctor, whose name could not be released for security reasons, said, "one was found hoarding medication in one. I think it was a leg prosthesis".
He said the prisoner had about 15 prescription pain pills, which "probably couldn't have killed him" but would have knocked him out, and several others had one or two pills hidden in the waistbands of their pants.
The doctor said up to 20 per cent of the detainees received medicines such as blood pressure medication, sleeping pills, pain killers and anti-anxiety pills.
The two who overdosed had swallowed a combination of medicines apparently obtained from other prisoners because none had been prescribed for them.
Under procedures implemented after the overdoses, a military guard now accompanies a soldier who distributes pills on the cell blocks and watches as each prisoner puts the pill on his tongue, swallows it with water and then opens his mouth and his hands to show he has not hidden it somewhere.
"I think it's as good as it can be without putting it down the throat myself," the doctor said.
Two Saudis and a Yemeni hanged themselves with clothes and bedsheets in their cells on June 10.
They were the first prisoners to die at Guantanamo since the United States began sending al-Qaeda and Taliban suspects to the prison camp in 2002.
US military officials have implemented new measures at preventing suicides since the prisoners killed themselves.
The prisoners received new uniforms and new bed mattresses, and they are being watched more closely while taking medication.
The doctor said camp officials were trying to make the camp suicide-proof but that "I would say that's impossible, short of putting these guys in straitjackets".
Supreme Court due to make ruling
Meanwhile US Attorney General Alberto Gonzales has defended the treatment of prisoners at the Guantanamo, saying the administration did its best and that the courts would judge "whether or not we made the right call".
Speaking at the start of a Middle East tour, Gonzales said the US Supreme Court was due this week to rule in a case that challenged President George W. Bush's power to try them before war crimes tribunals.
"We made the best judgment ... based on court precedent, based upon our reading of the law, based upon our reading of the constitution," Gonzales said in response to questions at Tel Aviv University.
"At the end of the day, it is up to the courts to make the decision as to whether or not we made the right call," he said.
Guantanamo houses about 450 suspected al-Qaeda and Taliban prisoners. Only 10 have been charged before US military war crimes tribunals.
Amnesty International and other human rights groups have called for the closure of the prison camp, which has helped undermined international support for the US war on terrorism.