Substance D
Substance D (also called Death, Slow Death, D) is a fictional recreational drug used by characters in the Philip K. Dick novel, A Scanner Darkly.

# Composition of Substance D
Not much is said about the drug's chemical makeup, except that it is synthesised in a laboratory in an unknown location.  It is derived from a small, blue flower grown primarily in the Anaheim, California area in a large produce farm. Substance D can be cut with meth, cocaine, or seemingly any stimulant drug. , 

# Effects of Substance D
If taken in large enough amounts (and in a high enough frequency) Substance D can cause hallucinations, paranoia (if taken in in increased levels) psychosis (permanent, usually; exempli gratia: Charles Freck , Jerry Fabin ) and, in a few cases, death (hence its street name). , 
It is implied multiple times in Dick's novel that Substance D induces lethargy, indicating that Substance D is not a stimulant.  As is shown in the novel when Freck's ability to operate a car properly was hindered, Substance D apparently also impairs one's judgement (much like grain alcohol). , 
All throughout the novel, it is shown that long-term use of Substance D causes the two cerebral hemispheres of the brain to separate.  "Death" also inhibits the ability of the two halves of the brain to 'communicate' with each other.  This splitting of the brain (literally, right down the middle) ultimately causes a form of drug-induced Dissociative identity disorder (DID).  The main character in the novel, Robert Arctor (played by Keanu Reeves in the 2006 film), had his mind divided into one side that became "Bob Arctor" and the other side that became Fred (his identity as an undercover narc in the Anaheim Police Force).

# People who use Substance D
Those who regularly use Substance D (which appears to be most of Anaheim, California) are referred to as heads.  Those who don't, the users and addicts call straights. , , 
Philip K. Dick seems to separate the "straights" from the "heads," but makes it very nebulous as to who is dealing the drug.  According to the novel's assertion of it, the straights don't use Substance D, but may deal it, and the heads don't usually deal, but they do use it (and buy from other heads or even straights).

# The legality of Substance D
Substance D is illegal for anyone to have and/or sell, but not to produce.  This is evidenced by the fact that the California Police will set up an undercover sting operation to catch the dealers and those who possess it, but never go after the lab that manufactures it or the company that runs the lab. 
Dick also mentions in A Scanner Darkly that the police don't often arrest "small-time local dealer," such as Donna Hawthorne (played by Winona Ryder in the 2006 film).   However, despite all this, the police will " such persons automatically."
From the character of Charles Freck's view-point, California is a "fascist police state."

# Forms in which Substance D is sold
Substance D is sold in the following forms:
- "Tabs" (tablets) 
- Liquid (for injection, via hypodermic needle) 
Substance D (in the book) is only bought over the phone (called "buy calls") by way of a payphone, since the California Police bug everyone's phone lines.
One would call their dealer to arrange a meeting-spot to buy the drug, but even the pay phones are tapped. 
In the film, Substance D is purchased in the same way, only with a cellular phone (since the film was set in the year 2013; the book was set in 1994).  Cell-phone calls were monitored, so buyers kept their 'drug-talk' discrete.