An expert who found traces of drugs in Slobodan Milosevic's blood before his death says they may have neutralised treatment for his heart conditions.
Full test results are awaited, but the comments by toxicologist Donald Uges raise questions over what caused the heart attack that killed Mr Milosevic.
The ex-Yugoslav leader, on trial at The Hague war crimes tribunal, had alleged that he was being poisoned.
The tribunal says Mr Milosevic received competent medical supervision.
Russia, home to Mr Milosevic's widow and son, says it does not trust the autopsy report and wants its doctors to examine the body.
Foreign Minister Sergei Lavrov said he had been disturbed by the UN tribunal's decision to turn down Mr Milosevic's request to undergo treatment in Russia.
Serbian President Boris Tadic told the Associated Press news agency that Mr Milosevic should have been granted a "higher level of health care" and that the tribunal was responsible for Mr Milosevic's death.
Mr Milosevic's lawyer said he had asked Serbian authorities to allow the body to be returned to Belgrade for a funeral. Serbia has yet to respond.
The former leader was found dead in his cell at the UN tribunal on Saturday morning.
Mr Uges said he tested Mr Milosevic's blood two weeks ago at the tribunal's request because his blood pressure was not falling.
Mr Uges said rifampicin, a powerful antibiotic used to treat leprosy or tuberculosis, was found.
Rifampicin would neutralise the effects of heart drugs, he said.
Mr Uges added that he believed Mr Milosevic took the drugs himself to try to prove his medical care in The Netherlands was inadequate and secure a "one-way ticket to Moscow".
However, the presence of rifampicin or other drugs has not yet been confirmed by the post-mortem toxicological tests - the results of which have still to be released.
Mr Milosevic's lawyer, Zdenko Tomanovic, has said the former president feared he was being poisoned.
On Monday, the Russian foreign ministry said it had received a handwritten letter from Mr Milosevic days before his death complaining of "inadequate treatment by doctors" at The Hague.
In the letter he renewed his plea - rejected by the tribunal - for treatment in Moscow.
Sources close to the tribunal suggest Mr Milosevic may have deliberately mixed or missed the dosages for his bad heart and high blood pressure.
Mr Milosevic's body is to be released on Monday or Tuesday, said Mr Tomanovic.
Serb authorities fear a funeral there would be embarrassing
The former leader's son, Marko, has been granted a visa by the Dutch embassy in Moscow to travel to the Netherlands to collect his father's remains.
His mother, Mirjana - known as Mira - Markovic, earlier told Serbian newspaper Vecernje Novosti she had not decided on the location for the burial.
But she added: "If it was only up to me to decide it would be Pozarevac" - Mr Milosevic's birthplace 80km (50 miles) east of the capital, Belgrade.
She faces charges of abuse of power if she returns to Serbia, but Serbian President Boris Tadic has ruled out a pardon, and said a state funeral for Mr Milosevic would be "completely inappropriate".
Mr Milosevic's daughter Marija had said he should be buried in Lijeva Reka - the family's ancestral home in Montenegro.
Authorities in Serbia and Montenegro fear a burial there could spark an embarrassing show of support from the former president's hardcore supporters.
But Mr Milosevic's Socialist Party said it would try to bring down Serbia's minority coalition government - which relies on its votes - if there were no funeral there.
Mr Milosevic was charged with war crimes and crimes against humanity for his alleged central role in the wars in Bosnia, Croatia and Kosovo during the 1990s.
He also faced genocide charges over the 1992-95 Bosnia war, in which 100,000 people died.