Cotler surprised people last week when he said publicly that he believed he was poisoned while on a visit to Russia. He revealed the information after it was announced that he was one of 13 Canadians, including five MPs, who are blacklisted from visiting Russia, a retaliatory move for Canada imposing sanctions following Russia's annexation of Crimea.
It is impossible to prove if he was deliberately slipped a toxic substance, because no investigation was conducted following his trip to a Moscow emergency room where he vomited blood. But over the years, he has become more convinced that it wasn't illness or food poisoning that resulted in his hospital visit while in Russia.
In June 2006, the MP for Mount Royal was part of an official Canadian delegation that included former Conservative MP Stockwell Day and NDP MP Joe Comartin.
It was a late spring afternoon when Cotler arrived in Moscow and checked into the elegant Baltschug Kempinski hotel, just steps from the Kremlin and Red Square.
That night, Cotler and Comartin had dinner at a restaurant. "We ate mostly the same thing," Cotler said during an interview in his office on Parliament Hill.
"I think he had something a little different for the appetizer. It was a very pleasant evening, so we sat outside," said Comartin, speaking from his riding in Windsor, Ont.
"I began to feel ill," Cotler said. "Later that night, when I was back in my hotel, I felt sicker almost than I ever had before in my life. I began to throw up blood."
Hotel sent cleaners
Cotler said he called the hotel's front desk and asked for a doctor. Instead, cleaners were sent to his room.
He then phoned the Canadian Embassy, which sent a doctor to his room. The Russian doctor accompanied Cotler to the European Medical Centre, a private hospital that caters to foreigners.
What the Canadian government says about the incident is summed up in an email from foreign affairs department spokesperson Jean-Bruno Villeneuve. "The Canadian Embassy in Moscow assisted Mr. Cotler in getting medical assistance when he became ill during his visit to Russia in 2006."
Cotler's medical bill from his hospital tests details a variety of blood tests, many to evaluate the functioning of his liver and pancreas, as well as X-rays of his abdomen and an IV. He was charged almost 17,000 rubles or $622, but the invoice, in English and Russian, contains no diagnosis.
"I didn't know what it was because they only spoke Russian. Something was mentioned about a poison, but I didn't know. I did not pay any attention to it, other than I felt sick," Cotler said.
"It is certainly not beyond the pale at all this was an intentional poisoning. Irwin has certainly been a thorn in their side for quite some time, going back to the Sharansky case," Comartin said.
Cotler, a long-time human rights lawyer, has represented Russian dissidents Natan Sharansky and Andrei Sakharov.
He has also advocated for accused spy and environmentalist Vladimir Nikitin, and recently took up the cause of Sergei Magnitsky, whose death in a Russian prison in 2009 brought charges of human rights violations against the Vladimir Putin regime.
Connecting the 'dots'
Cotler said he began to "connect the dots."
One of his former classmates from Yale Law School, Luzius Wilhaber, the former president of the European Court of Human Rights, also claimed he had been poisoned when he visited Russia, shortly after Cotler's trip.
Wilhaber told a Swiss newspaper he became violently ill and was hospitalized after a three-day Moscow visit in October, 2007. Wilhaber had been singled out by Russia for upholding complaints from Chechen human rights activists.
In 2004, the Ukrainian president and leader of the country's Orange Revolution was poisoned in a restaurant with dioxin, a component used in the herbicide Agent Orange. Viktor Yushchenko survived, but his face was severely disfigured.
In 2006, former KGB agent Alexander Litvinenko was dosed with the radioactive substance polonium, which was thought to have been placed in a cup of tea he drank in a London restaurant. He died three weeks later in a London hospital.
Cotler's suspicions about his own poisoning were heightened when he was chatting with Russian Embassy officials in Ottawa in 2010. He says they asked him why he hadn't visited Moscow lately. He said his reply was, "Well the last time I was there I was poisoned."
He said the Russians responded, "Sorry about that. It was a mistake. It won't happen again."
The Russian Embassy did not reply to an email from CBC News asking about Cotler's allegation of poisoning.
Ray Boisvert, a former assistant director of the Canadian Security Intelligence Service (CSIS), said in an email that poisoning was a "signature assassination" method of the KGB (now known as the FSB or SVR).
"One would recall the famous umbrella caper in London, where a Bulgarian defector was stabbed with a poison pellet in the 1980s," he wrote.
Boisvert thinks Cotler was a much smaller target.
"In dealing with persons who are simply 'annoying' the regime, cyber attacks by FSB or SVR proxies have been the preferred counter move," he wrote.
But Comartin, a lawyer as well as a politician, wonders why the blood tests administered to Cotler weren't screened for toxins, or, if it was a case of food poisoning, why Cotler wasn't questioned by public health officials.
Stockwell Day, whose portfolios as a cabinet minister included public safety, said, "Nothing is ever ruled out in situations like that."
Cotler, a former minister of justice, has announced he won't be running in the 2015 election.