When Soviet scientists developed a new group of nerve agents in the 1970s and ’80s, they called it Novichok, or “newcomer.” The lethal cocktail acts as a cholinesterase inhibitor, blocking an enzyme the nervous system needs to function. The poison disrupts communication between nerves and muscles.

The Organization for the Prohibition of Chemical Weapons added Novichock to a list of banned chemical toxins in 2019 after Britain accused Russian agents of using it to poison a former Russian operative and his daughter a year earlier. The Kremlin denied the allegations.

Berlin’s Charité hospital said in late August that another Russian dissident, prominent opposition figure Alexei Navalny, was poisoned on Aug. 20. Days later, a German government spokesman said Navalny had been dosed with a Novichok-like nerve agent, citing “unequivocal” evidence. Labs in France and Sweden confirmed the findings.

Navalny went on to spend more than two weeks in a medically induced coma in Germany, where he was flown several days after the poisoning. He is now recovering and has vowed to return to Russia.

On Thursday, his team said traces of nerve agent had been found on a water bottle recovered from his Siberian hotel room.

The Kremlin has again denied it was involved. But many analysts say a clear pattern of attacks against opponents of Russian President Vladimir Putin suggests otherwise.

What are cholinesterase inhibitors?

Cholinesterase inhibitors, also called anti-cholinesterases, are a compound of chemicals for which scientists have found many uses, including chemical weapons, pesticides and Alzheimer’s therapies, according to Reuters.

Improperly inhaling, ingesting or coming into eye or skin contact with the chemicals can be dangerous. Carbamates and organophosphates, both pesticides, use cholinesterase inhibitors to neutralize the nervous system of insects. The compounds can also be harmful to humans if ingested.

Novichok and sarin are among a subgroup of compounds known as nerve agents. They are many times more potent than the pesticides. Sarin was on the original 1997 list of toxins banned by the OPCW, a global chemical weapons watchdog.

What do they do to the body?

Cholinesterase is an enzyme the nervous system needs to regulate the body’s basic function, including the heart and lungs. Patients poisoned with compounds that block the enzyme can experience multisystem flare-ups, seizures, paralysis and sometimes death, usually from respiratory failure.

For example, the highly volatile agent sarin, which Syrian President Bashar al-Assad is accused of using in multiple deadly chemical weapons attacks against Syrians in spring 2017, is a colorless, odorless and tasteless liquid that can spread through fluids including water or through the air as a gas, according to the Centers for Disease Control and Prevention. Once ingested, the chemical disrupts the function of glands and muscles. That can lead to symptoms including a runny nose, headache, blurred vision and trouble breathing. But heavier exposure, which can be fatal, means loss of consciousness, paralysis, convulsions and respiratory failure.

Novichok is potent in part because it can work fast, sometimes within 30 seconds, depending on the dose, and can be inhaled, ingested or absorbed through the skin, according to the BBC. Those who may have been exposed should change their clothes and wash their skin, according to experts.

How do doctors identify and treat Novichok poisoning?

Nerve agents can be hard to detect.

A “broad analysis has been initiated” to determine the substance with which Navalny was poisoned, Berlin’s Charité hospital said in a statement in August. But doctors said they had confirmed through tests that a toxic chemical blocking cholinesterase was the cause of Navalny’s illness.

Doctors provided Navalny with atropine, a medicine used to treat some forms of nerve agent and pesticide poisonings. Athene is another nerve agent antidote that can help counteract the poison but is not a cure.

Navalny was placed in a medically induced coma for treatment, but there was “no acute danger to his life,” the hospital said. “Longer-term effects, especially in the area of the nervous system, cannot be ruled out.”

Why do victims sometimes survive?

Killing with nerve agents is an imperfect science and depends on dosage and method of exposure. “The practical difficulties in disseminating any type of poison will introduce a wide margin of error in dosing,” Robert Chilcott, a toxicology expert at the University of Hertfordshire, wrote in Ars Technica.

Absorption through the skin can be hard to modulate, and if Navalny consumed poisoned tea, as some initial evidence suggested, or poisoned water from a bottle in his hotel room, it would be difficult to predict how much he might drink.

The point of poisoning may not be its foolproof efficacy.

“If you’re a regime that is willing to kill enemies at home and abroad, you have to decide on your priorities: ease, subtlety or theatricality,” Mark Galeotti, director of the London-based firm Mayak Intelligence, told The Washington Post. “For the second and third, poison is often a good bet."

Has Russia been accused before of using nerve agents against critics?

Navalny’s spokesperson accused Russian authorities of deliberately poisoning him, charges the Kremlin denies. But well-known critics of Putin have been the victim of suspected or confirmed poisonings.

In 2018, former Russian spy Sergei Skripal and his daughter were hospitalized after being poisoned with Novichok in their home in Salisbury, England. They survived, but a woman unconnected to the pair who came in contact with the chemical compound died. Britain blamed Russia for the attack. The Kremlin denied responsibility.

Pyotr Verzilov, an associate of the Pussy Riot protest group, sought medical care in Germany in 2018, after he became ill from a suspected poisoning. He blamed it on Russian military intelligence. Doctors in Berlin found no traces of poison in his system, though they said they could also identify no other possible explanation for his symptoms.

“My symptoms in the first hours of poisoning were VERY similar to what is happening with Navalny now,” Verzilov tweeted in August.

This report has been updated.