AKA the "lethal" pill. Pill containing cyanide issued to agents who would prefer to take their own life rather than submit to torture upon capture.

Some varieties on this theme are contained in hollow capped teeth that can be worked loose with the tongue and bitten down on. The tooth is (supposed) to pass harmlessly through the digestive tract if swallowed whole (as, for instance, during sleep). Another form was a needle, which was held up as incriminating evidence in the case of Francis Gary Powers, although in his case, the poison used was curare.

This idea was used in Star Trek (although amygladin was probably the idea, not HCN as such) and parodied in the movie M*A*S*H in the Suicide is Painless scene as "The Black Capsule" (da-da-da-DAH...).