Explainer: Can you recover from nerve gas poisoning?
As a former Russian spy and his daughter try to come back from a chemical attack, toxicologists warn that they may never be the same
As poisoned Russian double-agent Sergei Skripal was said to be on the mend on Friday, toxicology experts warned that the nerve agent used on him could leave long-term damage.
This is what we know.
Britain contends that the military-grade nerve agent that poisoned the 66-year-old ex-spy and his daughter Yulia, 33, was of the Novichok family developed by the Soviet government towards the end of the cold war. Russia denies any responsibility.
Nerve agents are the most toxic known chemical warfare agents. They affect the central nervous system by interrupting communication between the brain, the main organs and muscles.
Many people stop being able to breathe and asphyxiate, or suffer heart failure.