The Russian hospital which has treated opposition leader Alexei Navalny for an apparent allergic reaction says doctors did not find any traces of poison in his system.
Mr Navalny, 43, was taken to hospital over the weekend but is now said to be in a satisfactory condition and has been returned to prison.
The opposition leader is serving a 30-day sentence for violating strict protest laws and calling for unauthorised demonstration in Moscow which led to the arrests of more than 1,000 people.
Mr Navalny questioned the hospital’s assertion he had suffered an acute allergic reaction, saying: “I have never had an allergy. Not to food or pollen or anything else.”
Writing in a blog post from prison, he said: “At night, I woke up with a hot and prickly face, ears and neck. I felt like I’d had my face rubbed with glass wool.”
“I had the thought, maybe I’ve been poisoned.”
The politician said his wife suffers from allergies so he knows what to expect.
Mr Navalny was taken to hospital on Sunday morning with a swollen face, eye problems and rashes on his body.
Doctors diagnosed him with “contact dermatitis”.
His lawyer and personal doctor were suspicious of the official diagnosis and said they could not rule out that he had been poisoned.
Anastasia Vasilyeva, a doctor who has treated Mr Navalny in the past, said she had been able to speak briefly with him and see him through the crack of a door.
“We cannot rule out that toxic damage to the skin and mucous membranes by an unknown chemical substance was inflicted with the help of a third party,” she said.
The doctor was also concerned Mr Navalny was discharged before the necessary tests could be run on him and said the suspected poisonous substance could still be in his prison cell.
His lawyer, Olga Mikhailova, also said the incident was “poisoning by some kind of unknown chemical substance”.
Mr Navalny has been a staunch critic of the Kremlin and described President Vladimir Putin’s ruling party as “crooks and thieves”.