MOSCOW — Without the slightest premonition of any issues with his health, Pyotr Verzilov, a Russian opposition activist, suddenly fell violently ill two years ago and slipped into a coma, a common problem for opponents of the Kremlin.
Mr. Verzilov, who is known for staging antigovernment performance art, displayed the same, mysterious symptoms that struck Aleksei Navalny, Russia’s most prominent opposition politician, on Thursday as he was on a flight to Moscow.
“I was in exactly the same condition,” Mr. Verzilov said in an interview Thursday with Rain TV, an independent Russian television station, of his monthlong illness in 2018.
Mr. Verzilov was kept alive on a ventilator and later flown to Germany for treatment. Though doctors found no trace of poison, he said that he is convinced it was the cause, and that the Kremlin was responsible.
Poison, though redolent of medieval intrigue, has been a favored tool of Russian intelligence agencies for more than a century. And critics of the Kremlin and independent analysts say the weapon remains in use today. While other countries, including the United States and Israel, have targeted killing programs, they are strictly limited to counterterrorism efforts. Russia, by contrast, has been accused of targeting a wide variety of opponents both at home and abroad.
The Soviet Union operated a secret laboratory to research tasteless and untraceable poisons that were tested on condemned gulag prisoners, security service defectors have said.
After a series of assassinations and attempted assassinations of dissidents, journalists, defectors and opposition leaders in Russia and abroad over the past two decades, researchers have concluded the post-Soviet government has turned to its poison arsenal as a preferred weapon.
Substances that have been identified or suspected in poisonings blamed on the Russian government include radioactive polonium-210; heavy metals; gelsemium, a rare Himalayan plant toxin; and Novichuk, a military nerve agent lethal to the touch.
Lacing a meal or cup of tea — the last substance Mr. Navalny is said to have consumed at an airport cafe before falling ill — with poison is simple and requires no special training, Gennadi V. Gudkov, a former opposition member of Parliament and onetime colonel in the K.G.B., said in a telephone interview on Thursday.
“It is easy, and easy to cover your tracks,” he said. “Any person can use poison.” Poisons can be intended either to kill or to incapacitate a person with a long and unpleasant illness, he said.
Ukraine’s former pro-Western president, Viktor A. Yushchenko, for example, was left with his face disfigured after a poisoning with the industrial pollutant dioxin — most likely concealed in a meal of boiled crayfish. Mr. Yushchenko attributed the poisoning to Russian agents.
The Kremlin has for years regarded Mr. Navalny as an enemy because of his investigations into graft by officials. He has been harassed and jailed numerous times, but only for short periods.
Mr. Navalny was rushed to a Siberian hospital after his flight made an emergency landing on Thursday because of his sudden illness. Doctors have not announced a cause.
The state-owned news agency Tass quoted an unidentified law enforcement source as saying that the authorities were not yet considering the possibility of a deliberate poisoning. Mr. Navalny’s personal doctor, Yaroslav Ashikhmin, said he has not seen Mr. Navalny since the illness began, so he could not say whether poison was the cause. But, he added, “it looks like it.”
If Mr. Navalny were poisoned before or during his flight, it would not be the first time an opposition figure was targeted while sitting in the controlled environment of a commercial airplane in Russia.
In 2015, the opposition activist Vladimir Kara-Murza fell into a weeklong coma in Moscow. He later said he believed he had ingested a poison during the in-flight service on an Aeroflot plane.
His symptoms included swelling in his brain and kidney failure. His wife, Yevgenia, recalled that his arms and legs took on a blue hue, an alarming, almost cartoonish reaction to poison.
Mr. Kara-Murza said he was poisoned and survived a second time in 2017 while traveling in Russia to show a documentary about another Russian politician, Boris Y. Nemtsov, who in 2015 was shot and killed on a bridge in Moscow.
In 2004, the opposition journalist Anna Politkovskaya was also poisoned on a domestic flight operated by another airline, Karat. She drank a poisoned cup of tea, she said. She survived but two years later was shot and killed in her apartment elevator.
Toxins lethal to the touch have also come to light. The Arab-born terrorist Ibn al-Khattab died in 2002 in his mountain hide-out in Chechnya after opening a letter laced with a nerve agent.
Some toxins may have also slipped from government arsenals and into the organized crimes wars in Russia in the early post-Soviet period. In 1995, for example, a Russian banker, Ivan K. Kivelidi, died after coming in contact with a poison deadly to the touch. The cause of his death might have remained a mystery had his secretary not also died of the same symptoms, apparently because the poison had been spread on an office telephone handset.