According to this list of drug slang terms from a declassified DEA report, 51 is slang for crack cocaine.
Checking Green's Dictionary of Slang confirms the term (listed as fifty-one) and lists the etymology as unknown. However, I noticed that GDoS has an entry for "5150":
(US black/prison) someone in need of mental health treatment; an eccentric, a crazy person.
Etymology:
[police code, an insane person is annoying the public]
Is there any evidence of a potential relationship between the slang 5150 meaning "an eccentric/crazy person" and 51 meaning crack cocaine or tobacco laced with crack cocaine? Are there alternative theories?
The only alternative explanation I found is on Urban Dictionary; I can find no reputable source to back it up, and in fact, it even has a low rank on UD so I mention it only as a possible starting point for investigation:
a joint with $1 worth of weed and $5 of freebase.
Best Answer
What might be called 'disambiguation' in other contexts is, in the context of drug use and abuse terminology, more on the order of 'deobfuscation'.
The mixture of '151', '5150' and '51' has some stupefying characteristics in common with the most usual reference of '51', that is, a
The use of '51' in this sense is either an easy-to-speak-while-jonesing street slang obfuscation-via-truncation of '151' in the sense of 'cocaine', a term itself shortened from the ICD-10 (International Classification of Diseases) diagnostic code F14.151 (late 1980s and early 1990s), which refers to "cocaine abuse with cocaine-induced psychotic disorder with hallucinations", or the similar shortening of '5150', which last refers to the 5150 section of the California Welfare and Institutions Code, or both. That section of the California WIC authorizes the temporary detention of "Mentally Disordered Persons for Evaluation and Treatment" if they are deemed "a danger to others, or to himself or herself, or gravely disabled".
Most likely is that the coincidence of '51' appearing in both the ICD-10 diagnostic code and the California WIC section played a part in the street adoption of the term.
Other influences may include '5-1' in Roman numerals being the heart of EVIL (VI), symbolic use of the '5150' code by heavy metal bands and rappers of the 1980s and 1990s (for example, Van Halen's studio, song and album of that name), and the confluent history of crack cocaine use in the US, which peaked during the same time period (1980s and 1990s).