Organophosphates

Organophosphates, including the chemical weapon Sarin Gas, work by inhibiting your muscle's ability to relax. Your muscles basically constrict and can't unconstrict, causing what feels like a muscle cramp through your entire body - your arms, your chest, your eyes, your tongue, everything. It most frequently kills via asphyxiation, because you can't exhale. Surviving means a permanent, irrecoverable loss of motor function, even with rapid medical treatment.