Ukrainian opposition leader Viktor Yushchenko says he's "very happy to be alive" after doctors verified he was poisoned with dioxin that could have been put in his food.

With his wife, Kateryna, by his side, the 50-year-old presidential candidate spoke briefly to reporters Sunday before leaving a private Vienna clinic where he was diagnosed.

Yushchenko first fell ill in September and was rushed to hospital. He resumed campaigning, but his mysterious illness had left his face pockmarked and ashen.

On Saturday, doctors at the Rudolfinerhaus clinic said his internal organs now seem to be fine and they expect his face will heal in two to three years. He was deemed well enough to continue his campaign for the new presidential runoff.

But they said a slightly higher amount of dioxin would have killed Yushchenko. They said there was 1,000 times the normal amount of dioxin in the politician's blood and tissue.

Dioxin is a toxic substance that is classified as a human carcinogen. It can damage the liver, thyroid, intestinal track and nervous system.

Dr. Arnold Schecter of the University of Texas in Dallas told CBC Newsworld you can treat the symptoms of dioxin poisoning, but there's no method for removing it from the body.

Dr. J.J. Ryan of Health Canada told Newsworld most dioxins have a half-life of five to 10 years.

Yushchenko had accused Ukrainian authorities of trying to poison him ahead of Ukraine's presidential vote, an allegation they denied.

However, Ukrainian prosecutors now say they will re-open their investigation into the poisoning.

On Dec. 3, Ukraine's Supreme Court invalidated the Nov. 21 runoff vote, in which the sitting prime minister, Viktor Yanukovich, was declared the victor by state election authorities. The court ordered a new vote for Dec. 26.

Before checking out of the Rudolfinerhaus clinic, Yushchenko praised the decision of thousands in Ukraine to take to the streets to protest the outcome of presidential elections.

"We haven't seen anything like that for the past 100 years," he said. "I think it would be appropriate to compare this to the fall of the Soviet Union or the fall of the Berlin Wall."