During the Cold War, both Washington and Moscow built up large quantities of chemical weapons, including VX.
But after signing the Chemical Weapons Convention, which banned the production, stockpiling and use of chemical weapons, the U.S. says it has destroyed all of its arsenal and Russia has pledged to do the same by 2020.
Elsewhere, the spread of VX is believed to be relatively contained, mainly because it takes a sophisticated laboratory to produce.
Saddam Hussein was accused successfully weaponizing VX in the 1980s, before using it against Iranian forces and the Kurds.
A decade later, the Japanese cult Aum Shinrikyo killed 12 people on the Tokyo subway using the less-toxic nerve agent sarin. The group also killed one person using VX.
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The apparent assassination of Kim Jong Nam would add North Korea to this inglorious list.
Although the recent focus has been on its nuclear arsenal, the country is believed to possess between 2,500 and 5,000 tons of chemical weapons, according to the Nuclear Threat Initiative, a non-profit organization based in Washington, D.C.
"Nerve agents such as Sarin and VX are thought be to be the focus of North Korean production," it said, although Kim's death would be the first known occasion where the country has actually deployed it.
North Korea, Egypt and South Sudan are the only countries in the world that haven't signed the Chemical Weapons Convention.
Unlike sarin, which is usually deployed as a gas, VX is very slow to evaporate and is therefore usually found as a viscous liquid, similar in texture to motor oil or honey.
In this state, it's highly toxic when it comes into contact with skin.
"You need a microscopic amount to kill one person, which is what happened to Kim Jong Nam," said Bretton-Gordon, the chemical weapons expert.
It's likely that Kim at experienced pinpointed pupils, a runny nose, and nausea, before finding it hard to breathe and feeling his heart racing.
He probably then had loss of bladder and bowel control, convulsions, seizures, and finally death while on the way to the hospital just minutes later.
There are antidotes, such as the medication Atropine, which the French military were issued with after the Paris attacks because they feared ISIS would attempt to use VX. But this needs to be administered almost immediately to be effective.
"VX acts so quickly that victims would have to be injected with the antidote almost immediately to have a chance at survival," according to the Council on Foreign Relations.
Bretton-Gordon added that "Kim Jong Nam had absolutely no chance at all."
VX also featured in the 1996 action thriller "The Rock."
Although the symptoms displayed in the movie — the liquid evaporating quickly and burning the skin — are inaccurate, and more closely associated with chlorine.
This might be down to VX's chemical properties. It doesn't evaporate very quickly, so people have less chance of breathing it into their lungs.
There may have been one other casualty, however: Malaysian officials say that one of the two women who allegedly attacked Kim vomited after the incident.
Furthermore, police say the two women washed their hands straight after. This tallies with guidelines from the U.S. Army, saying that "a solution of common household bleach and water, followed by water rinse, can be used to decontaminate the skin where contact was made with VX."
Assuming the chemical was made in North Korea and not Malaysia itself, getting such a potent substance past security may have been relatively easy.
Bretton-Gordon explained that when he gives talks about the danger of chemical weapons, he uses a thimble-sized container of honey as a prop to demonstrate how little of the liquid is needed to pose a threat.
"I carry it in my hand luggage all over the world and no one has said anything — why would VX be any different?" he said.
For the past decade, international focus has been on North Korea's ability to deliver a nuclear weapon on a long-range missile — something analysts predict is some way off.
But Kim's assassination may reveal a far more short-term threat: that it could arm a missile with VX and fire it at a city or a military base.
Previously, one thing that's reassured experts about terrorists' ability to use VX is that it's very hard to produce. "It's not something you can knock up in your back shed," according to Bretton-Gordon.
But this also means that the substance used to kill Kim was likely cooked up in a government-level laboratory — another indication that Pyongyang was behind the attack.
"If they were behind this then it means a nation state has taken a weapon of mass destruction into another country and used it," Bretton-Gordon added. "It will be very interesting to see what the international reaction will be."
Alexander Smith is a senior reporter for NBC News Digital based in London.