They used the same drug on Skripals, and both of them survived. The issues is that the drug is very tricky to dose right, especially since they seem to aim for

They used the same drug on Skripals, and both of them survived. The issues is that the drug is very tricky to dose right, especially since they seem to aim for killing people slowly and with some (now obviously failed too) plausible deniability. If they would just give people a sure dose that would kill them instantly, it would be obvious what they died from and where.