Asked whether David Zhvaniya, a parliamentarian from the pro-presidential Our Ukraine-People's Self-Defense bloc, could have been involved in poisoning him, Yushchenko said, "I think he could."
He did not elaborate on his opinion. Zhvaniya, who was born in Georgia, has been involved in a long-running citizenship row with Ukrainian authorities.
Yushchenko became seriously ill in early September 2004, the day after attending a dinner with Ukrainian security services leaders.
He suffered from back pain, acute pancreatitis, and nerve paralysis on the left side of his face. After the illness, his face became heavily disfigured - grossly jaundiced, bloated, and pockmarked.
Zhvaniya, who also attended the reception, earlier said Yushchenko was not poisoned and that his facial disfigurement was the result of "food poisoning". Yushchenko's doctors have dismissed this theory.
Pro-Western Yushchenko came to power after the "Orange Revolution," a popular bloodless uprising in Ukraine in late 2004. Zhvaniya was subsequently appointed emergencies minister.
However, after Ukrainian authorities questioned his right to citizenship, saying he had lied in an application about the amount of time he had spent in the country, Zhvaniya voiced his allegations.
"It was common food poisoning. The diagnosis was made on the first day. This kind of poisoning happens a lot," he said.
He said that the Orange coalition had invented the poisoning story to gain the sympathy of the Ukrainian people. Yushchenko, who has often claimed to know the identity of the people who poisoned him, has denied the allegations. He is due to stand for reelection next year.
Zhvaniya, who has appealed to the European Court of Human Rights over the citizenship row, was summoned on Thursday for questioning by the Prosecutor General's office.