Russian opposition politician Alexei Navalny fell into a coma after he collapsed in an aeroplane toilet on Thursday, prompting his allies to suspect poisoning.
If true, he would not be the first prominent, outspoken Russian to be the target of a toxic attack.
Here are some other people who have criticised the Kremlin and then fallen victim to suspected poisonings:
A former agent for the KGB and post-Soviet successor agency FSB, Lieutenant-Colonel Alexander Litvinenko defected from Russia in 2000 and fled to London, where he fell violently ill six years later after drinking tea laced with radioactive polonium-210.
He died after three weeks. A British inquiry found that Russian agents had killed Litvinenko, probably with President Vladimir Putin’s approval. Russia denied any involvement.
Before his death, Litvinenko told journalists that the FSB was still operating a secret Moscow poisons laboratory dating from the Soviet era.
He was one of several former Russian intelligence officers to accuse Moscow of being behind the dioxin poisoning of Ukrainian President Viktor Yushchenko during his 2004 election campaign.
At the time of Litvinenko’s poisoning, he had been investigating the killing of Russian journalist Anna Politkovskaya three weeks earlier.
An investigative journalist, Politkovskaya had written critically about abuses by Russian and pro-Moscow Chechen forces fighting separatists in Chechnya – work that earned her repeated death threats.
In 2004, she fell severely ill and lost consciousness after drinking a cup of tea. She said she was deliberately poisoned to prevent her from covering the 2004 seizure of a school in southern Russia by Chechen separatists.
Two years later, Politkovskaya was shot to death outside her Moscow apartment building, a murder that drew widespread condemnation in the West. Five men were sentenced for carrying out the killing but no one was convicted for ordering it.
Opposition activist Vladimir Kara-Murza Jr was hospitalised with poisoning symptoms twice, in 2015 and 2017.
A journalist and associate of Russian opposition leader Boris Nemtsov, who was shot and killed in 2015 while crossing a bridge near the Kremlin, and oligarch-turned-dissident Mikhail Khodorkovsky, Kara-Murza nearly died from kidney failure in the first incident.
He suspects poisoning but no cause has been determined.
He was taken to hospital with a sudden, similar illness in 2017 and put into a medically induced coma.
His wife said doctors confirmed he was poisoned. Kara-Murza survived, and police have refused requests to investigate the case, according to his lawyer.
A Russian spy who became a double agent for the UK, Sergei Skripal fell ill in the British city of Salisbury in 2018.
Authorities said Skripal and his adult daughter, Yulia, were poisoned with the military-grade nerve agent Novichok. The two spent weeks in critical condition.
The UK put the blame squarely on Russian intelligence, but Moscow denied any role.
Putin called Skripal a “scumbag” of no interest to the Kremlin because he was tried in Russia and exchanged in a spy swap in 2010.
The UK charged two Russian men with the poisoning. They claimed they had visited Salisbury as tourists and denied any involvement in the attack, which came amid revelations about Russian interference in the 2016 US presidential campaign.
Pyotr Verzilov, a member of the Russian protest group, Pussy Riot, was placed in an intensive care unit after a suspected poisoning in 2018 and had to be flown to the German capital, Berlin, for treatment.
German doctors treating him said a poisoning was “highly plausible”. He eventually recovered.
Verzilov, his partner and two other Pussy Riot members had served jail time earlier that year for running onto the field during the World Cup final in Moscow to protest against excessive Russian police powers.
He has also served time on other charges that he calls politically motivated.