"This is not poisoning by spoilt food products," Valery Natarov said.
Gaidar's daughter Maria said her 50-year-old father and former acting prime minister started vomiting and fainted at a conference in Dublin Friday, and remained unconscious for three hours. Gaidar was taken to a hospital in Dublin and later transferred to Moscow.
Natarov said doctors will not be able to give a precise diagnosis by the end of next week, as they have requested additional information from the hospital in Ireland where Gaidar was first taken to.
Natarov said Wednesday that Gaidar's condition was "stable and noticeably improving."
Gaidar's fellow reformer of the early 1990s, Anatoly Chubais, who is now chief executive of Russia's electricity monopoly, drew a parallel between the illness and the recent killings of an investigative journalist and a former security officer.
"Yegor Gaidar was on the verge of death on November 24," said Chubais, who was first deputy prime minister under former president Boris Yeltsin.
However, the CEO of Unified Energy System said Russian authorities were not involved in the incident. "If this was true, Moscow would have been a far better place for it than Dublin," Chubais said.
The Kremlin's reputation has been overshadowed by two recent high-profile killings. Anna Politkovskaya, an investigative journalist and Kremlin critic, was gunned down in Moscow in October in an apparent contract killing.
Alexander Litvinenko, a security service defector who allegedly investigated the murder, died in London last week from suspected radioactive poisoning.
Following his death, Western media circulated a statement in which he blamed the Kremlin and President Vladimir Putin for his alleged poisoning. The Kremlin denied involvement.
Both Politkovskaya and Litvinenko were linked to Russia's fugitive oligarch Boris Berezovsky, who had vast clout under Yeltsin, but is now living in London with a British passport. Berezovsky is wanted in Russia on fraud charges and attempts to organize a coup.
"The deadly triangle - Politkovskaya, Litvinenko and Gaidar - would have been very desirable for some people who are seeking an unconstitutional and forceful change of power in Russia," Chubais said.