German Chancellor Angela Merkel said that Alexei Navalny, critic of the Kremlin - who is being treated in the intensive care unit of a Berlin hospital - was poisoned with the nerve gas "Novichok", in an attempt to kill him, which brings to mind stories of opponents of Russia who were subjected to similar operations.

Navalny's poisoning has prompted some politicians and diplomats in Germany to propose a European response regarding the "Nord Stream 2" gas pipeline.

Here are some details about previous incidents in which opponents of the Kremlin fell victim to poisoning, attempts to poison, or caused a stir after suddenly falling ill.

Sergey Skripal

A former Russian double agent, who passed secrets to British intelligence, was 66 years old when he was found with his 33-year-old daughter Julia, unconscious, on a sofa in front of a mall in Salisbury, England, in March 2018.

The two were hospitalized in critical condition, and British officials said they were poisoned by using Novichok, a group of nerve agents developed by the Soviet army in the 1970s and 1980s.

Russia denied any role in poisoning them, and said Britain was fomenting an anti-Russian mania.

Drug specialist Gary Stephens - a professor at the University of Reading - said these chemicals "slow down the heartbeat and narrow the airways, which leads to death by suffocation."

"One of the main reasons for developing these drugs is that their ingredients are not on the banned list," he added.

Scribal was poisoned by a group of nerve gases developed by the Soviet army in the last century (Al-Jazeera)

Vladimir Kara-Morza

A Russian opposition activist says he believes there were two attempts to poison him in 2015 and 2017.

A German laboratory later found elevated levels of mercury, copper, manganese and zinc in its body, according to medical reports reviewed by Reuters, and Russia denied any role in this.

Alexander Litvinenko

A former KGB officer and a prominent opponent of Russian President Vladimir Putin, British officials said that he died at the age of 43 after drinking green tea mixed with polonium-200 - an active and rare radioactive isotope. At the Millennium Hotel in London.

A 2016 British investigation concluded that Putin likely consented to the killing, and the Kremlin denied any role.

An investigation led by a prominent British judge found that former KGB guard Andrei Lugovoy and another Russia, Dmitry Kovtun, carried out the killing as part of an operation he said the Russian Federal Security Service (FSB), the heir to the KGB, likely ordered. And Litvinenko had fled Russia to Britain 6 years before he was poisoned.

Litvinenko (left) drank green tea mixed with a toxin, and a British investigation concluded that Putin had probably agreed to kill him (Al-Jazeera)

Alexander Preplichny

A 44-year-old Russian man was found dead near his luxurious home in a fenced residential town outside London, after going out for a jog in November 2012.

Preplettechny asked for asylum in Britain in 2009 after he helped a Swiss investigation into a Russian money laundering scheme, and his sudden death sparked speculation that he might be killed.

The British police ruled out that the death was criminal, despite doubts that he might be killed with a rare poison, and the investigation into his death has not yet revealed a conclusive result of the cause of death.

In a session prior to the investigation, it was said that traces of a rare toxin extracted from the gelsemium plant had been found in his stomach.

Preplychny had eaten a large plate of famous Russian soup containing acid, and the Kremlin has denied any role in his death.

Victor Yushchenko

A opposition leader of Ukraine, he was poisoned during a 2004 presidential election campaign in which he ran on a pro-Western slate contesting the pro-Moscow Prime Minister Viktor Yanukovych.

He said he was poisoned while having dinner outside Kiev with officials from the Ukrainian security service, and Russia denied any connection.

A thousand times higher than normal for dioxin was found in his body, and his face and body were disfigured by the poison, and he underwent dozens of surgeries.

Yushchenko won the presidency in the re-election after the Supreme Court of Ukraine annulled the results announced by Yanukovych's victory, amid mass protests he called the "Orange Revolution".

 Georgy Markov

A Bulgarian writer and journalist opposed to the Communist leadership in his country at that time, fled to the West in 1969, and died on September 11, 1978 after feeling a sharp sting in his thigh while waiting for a bus on London's Waterloo Bridge.

According to accounts related to the incident, Markov looked behind him and saw a man picking up an umbrella that had fallen to the ground, and the man muttered, "Sorry," before proceeding.

Markov died of a toxin believed to be "ricin" for which there was no antidote, and the KGB defectors accused him of killing him.