Milosevic, who died in jail last Saturday just months before a verdict in his war crimes trial was expected, had suffered from high blood pressure and a heart condition, which according to an initial autopsy explained his heart attack.
But speculation grew over whether Milosevic had deliberately tried to exacerbate his condition or whether he had been poisoned, as the 64-year-old suggested in a letter addressed to Moscow the day before he died.
Judge Fausto Pocar, president of the UN war crimes tribunal, told a news conference on Friday that "no indications of poisoning have been found".
There was evidence of his prescribed medication, but not in toxic concentrations, he added, stressing that the results were provisional.
Tribunal registrar Hans Holthuis confirmed that traces of rifampicin - a leprosy and tuberculosis drug that would have neutralised Milosevic's medicines for his existing health conditions - were found in a 12 January blood test.
Holthuis said he could not explain how the rifampicin was found in his blood or how unprescribed drugs, which the tribunal knew Milosevic was taking, may have been smuggled in.
"That is indeed the burning question, I have no ready answer to that."
But no trace of rifampicin was found in Milosevic's blood at the time of his death, Pocar said. The Dutch Forensic Institute told investigators that the drug disappears from the body quickly and said it was unlikely it was taken in Milosevic's last days.
The tribunal has ordered an external inquiry into the running of the detention centre, where Croatian Serb leader Milan Babic committed suicide a week before Milosevic's death.
The court had denied a December request by Milosevic to travel to Russia for heart treatment, ruling he could receive the best possible treatment in the Netherlands.
Milosevic was on trial for war
crimes and genocide
One medical expert who tested the January blood samples said he believed Milosevic knowingly took rifampicin to improve his case for going to Russia, where his wife, son and brother live.
They have blamed the court for his death, but Borislav Milosevic, the brother of the late Serbian leader, said in reaction to the latest tests: "If they say there isn't any [evidence of poisoning] then there isn't."
The tribunal on Friday confirmed it knew Milosevic had access to unprescribed medicines and that "contraband" had been found during a search but it defended its security programme.
Said Holthuis: "I think we acted in a proper way."
As the tribunal had granted Milosevic the right to defend himself he was given a "privileged setting" - an office with a telephone and computer - to meet with witnesses he called.
Holthuis noted: "Witnesses were not subject to the same rigorous search methods as other visitors."
Retired Russian Colonel-General Leonid Ivashov, one such witness, has said he passed medicines to Milosevic.
"He did not trust the prison doctors. The entire extent of his medical treatment was him taking medicines that were brought to him by his visitors," he told Narodnoye radio on 11 March.
"We, among others, took medicines with us when we went to visit him," Ivashov added.
Milosevic's body, which was flown to Belgrade earlier this week, was put on view on Thursday before a private weekend burial in Milosevic's provincial home in Pozarevac.
The man branded the "Butcher of the Balkans" had been on trial for four years charged with 66 counts of genocide, crimes against humanity and war crimes involving conflicts in Bosnia, Croatia and Kosovo that tore Yugoslavia apart in the 1990s.