1. Substance D is a fictional addictive amphetamine-like psychoactive drug, also known as "Slow
Death", "
Death" and "D", that is depicted in Philip
K. Dick's novel "A Scanner Darkly". It's derived from the fictional flower "Mors ontologica".
2. By extension, Substance D can refer to amphetamine (speed).
"What he sold me as '
candy' actually turned out to be 'Substance D'."
This is a more subtle way to refer to amphetamine in a conversation if you
don't want to attract too much
attention.