The poisoning supposedly occurred Sep. 5, 2004, when Yushchenko was running for presidency as leader of the pro-Western opposition against the incumbent pro-Russian authorities. The poisoning is believed to have drawn sympathy for Yushchenko and helped him win.
Following a dinner with Ihor Smeshko, then head of the Security Service of Ukraine (SBU) at the summer house of his deputy Volodymyr Satsyuk, the presidential candidate fell unwell and said later he noticed a metallic taste in his mouth. The SBU is successor to the KGB of the Soviet days.
Yushchenko claimed he was poisoned, and blood tests carried out shortly after confirmed the presence of a dioxin in the President's blood and tissues, leaving his face disfigured.
"The Yushchenko poisoning is among the biggest Ukrainian mysteries of the decade," Zenon Zawada, chief editor of the English language Ukrainian weekly Kyiv Post told IPS. "There is really a lack of consensus about what happened.
"Among Ukrainians the most common theories are that the one responsible is either the SBU, or that Russia was somehow involved, though those who do not support Yushchenko say he had an allergic reaction or food or medication poisoning."
Yushchenko claims to know who poisoned him, and hinted in September last year at the involvement of Ukrainian politicians. He says the three main suspects are in Russia.
Just a few days after the statement the Kommersant-Ukraina daily tracked down the names of three individuals Yushchenko was allegedly referring to, citing unnamed sources at the Prosecutor-General's office.
The daily named Satsyuk, the infamous host, Taras Zaleskyy, an aide to Satsyuk who was also present at the dinner, and Oleksiy Poletukha, also a former aide to Satsyuk who could have played a role in transporting the dioxin from Russia.
But the President refuses to go public with his information in order not to hamper the investigation that he claims is being "stalled."
After mercury vapour was found in the office of the Prosecutor-General's deputy Mykola Holomsha last December, some interpreted the attempt on his life as a warning to prosecutors to slow down the inquiry.
Holomsha, who is a potential candidate to replace the contested Prosecutor-General Oleksandr Medvenko, is also in charge of other high profile cases, such as the murder of opposition journalist Heorhiy Gongadze in 2000.
The criminal case on Yushchenko's poisoning was instituted by the Prosecutor-General's Office three weeks after the poisoning occurred, but changes in personnel have had a role in delaying the inquiry.
Stefan Schocher, an Austrian journalist who investigated the case in its early stage and was in Austria when Yushchenko received treatment in Vienna's Rudolfinerhaus clinic, told IPS he cannot be sure the poisoning occurred at all.
"Everything seemed to be a game," Schocher told IPS from Kiev. "It was completely unclear who was playing what, different agencies were organising press conferences at the clinic, and depending on who was organising it they said something different."
The uncertainty of the case was confirmed to Schocher following a conversation with a doctor from the Austrian hospital who treated Yushchenko.
"He told me the dioxin doses were given over a longer period of time and not at once, like Yushchenko claims. Strangely, the doctor later denied it, but he definitely told me this."
The Austrian journalist says that "if it was a dose given over a longer period, it must have been someone from Yushchenko's team."
Other international media outlets, such as the Telegraph, the Frankfurter Allgemeine Zeitung or Le Figaro reported on an atmosphere of intimidation at the clinic, especially against those who questioned Yushchenko's poisoning.
Medical director at the clinic Dr. Lothar Wicke received death threats after casting a shadow on the poisoning thesis, and resigned his job just one day after Yushchenko was to receive another round of tests.
Dr. Wicke told the Telegraph in 2005 that the death threats came from Yuschchenko's entourage because he had cast doubt on the poisoning diagnosis. He said his resignation was forced upon him after 25 years of working for the clinic.
The medical director, who has received the Cross of Honour First Class for Science and Art from late Austrian president Thomas Klestil, accused Yushchenko's entourage of putting heavy pressure on the clinic to publicly support the poisoning thesis, which it eventually did.
"The first two times Mr Yushchenko was examined, there was no evidence of poisoning whatsoever," Dr. Wicke told the Telegraph. "I was directly involved, and I can tell you that the Institute of Forensic Medicine in Vienna did not find any traces of poisonous agents in his blood. If there is no poison, there cannot be poisoning, and there was no trace of it whatsoever."
But the latest developments in the case show that the prosecution is focusing on the Russia-based suspects.
Yushchenko, who claimed to have personally asked Russian President Vladimir Putin to assist with the case, complained in September of silence from the Russian side, and implied that Moscow was hindering the investigation.
Russian media reacted with contempt at the insinuation, noting that Yushchenko's statements came only a couple of weeks ahead of the crucial Sep. 30 parliamentary vote. Russian media accused him of trying to boost his ratings by playing the 'anti-Russian card'.
But shortly after the President's complaints Russia promised to facilitate the inquiry, and a joint commission was set up.
By November Ukrainian Prosecutor-General Oleksandr Medvenko said the inquiry was focusing on individuals living in Russia, but praised Moscow's "active cooperation".
Medvenko says the dioxin in question was produced in Russia before 2004 but says it was never exported. The Prosecutor-General promises to solve the matter this year.
The dioxin which was allegedly used to poison the President is only produced in the U.S., U.K. and Russia, but the Ukrainian prosecution established that the dioxin had been manufactured in Russia.
Investigators in Ukraine are still awaiting samples of the Russian dioxin, and have put further questions to their Russian colleagues.
The Russian side invited Ukrainian investigators to attend tests in Russia, but the Ukrainian prosecution insists on a test held in Ukraine in compliance with its legislation.
Ukrainian officials say that with a test in Russia, the evidence could not be used in court. They have invited Russian experts to attend the test in Ukraine.
The Russian side says its legislation only allows the test to be carried out in its territory. But such a stand will do little to improve the big neighbour's image in Ukraine. "The less helpful Russia is, the more people suspect there was some involvement from Russia," Zawada told IPS.