Deputy chief doctor Anatoly Kalinichenko at Omsk hospital said that as of Friday, no traces of poison were found in Mr Navalny’s body.
Mr Navalny’s spokeswoman Kira Yarmysh posted a video on Twitter of Mr Kalinichenko speaking.
“Poisoning as a diagnosis remains on the back burner, but we don’t believe that the patient suffered from poisoning,” Mr Kalinichenko told reporters on Friday.
Doctors in the Siberian city of Omsk have also refused to authorise the transfer of Mr Navalny to a German hospital, Ms Yarmysh said on Twitter.
Mr Navalny remains in a coma in intensive care after a suspected poisoning that his allies link to his political activity.
“The chief doctor said that Navalny is non-transportable. (His) condition is unstable. Family’s decision to transfer him is not enough,” Ms Yarmysh said in a tweet.
Mr Navalny, 44, fell ill on a flight back to Moscow from the Siberian city of Tomsk on Thursday and was taken to hospital after the plane made an emergency landing in Omsk.
His team says a plane with all the necessary equipment is ready to take Mr Navalny to a German clinic.
Mr Navalny’s ally Ivan Zhdanov said on Friday that police found “a very dangerous substance” in Mr Navalny’s system, but officials refuse to disclose what it is.
Alexander Murakhovsky, chief doctor of the Omsk Ambulance Hospital No. 1 where the politician is being treated, told reporters on Friday that Mr Navalny’s condition is “somewhat improved”, but he was not stable enough for a transfer.
Mr Murakhovsky said doctors were still working on determining a diagnosis.
Ms Yarmysh also said in her tweet that “the ban on transferring Navalny is needed to stall and wait until the poison in his body can no longer be traced. Yet every hour of stalling creates a threat to his life”.
Like many other opposition politicians in Russia, Mr Navalny has been frequently detained by law enforcement and harassed by pro-Kremlin groups.
In 2017, he was attacked by several men who threw antiseptic in his face, damaging an eye.
Last year, Mr Navalny was taken to a hospital from prison, where he was serving a sentence following an administrative arrest, with what his team said was suspected poisoning.
Doctors said he had a severe allergic attack and discharged him back to prison the following day.
Mr Navalny’s Foundation for Fighting Corruption has been exposing graft among government officials, including some at the highest level.
Last month, he had to shut the foundation after a financially devastating lawsuit from Yevgeny Prigozhin, a businessman with close ties to the Kremlin.
The most prominent member of Russia’s opposition, Mr Navalny campaigned to challenge Mr Putin in the 2018 presidential election, but was barred from running.
He set up campaign offices across Russia and has been promoting opposition candidates in regional elections, challenging members of Russia’s ruling party, United Russia.
One of his associates in Khabarovsk, a city in Russia’s Far East that has been engulfed in mass protests against the arrest of the region’s governor, was detained last week after calling for a strike at a rally.
