I watched DTs take an 80mg Oxycontin(Back when they were still being made), crush it up and snort it. DTs WILL use. The Departments they work for will use any means necessary in certain areas. Its not always the "Right" way. Funny, another film with Patric, 'Narc', shows this very issue. Its ABSOLUTELY happens.

(It's V??e??i??n?? as in blood in the vein, not vain as he was arrogant and vain)
Many people who sell drugs don't demand to see proof that someone is not a cop as long as they have enough money.
In the 70's many cops did use drugs to "pass." But it's a lot more difficult today as many, if not most, depts require random drug testing for everyone but especially for those in Narcotics.
This film, although not perfect in a few ways, is still one of the best I have seen with epic performances by the sexy Jason Patric, JJL and the always gorgeous Sam Elliot.

Well, I thought it was like they said in this film, that they kinda lied/"simulated" drug use while often having to actually take drugs.
To answer your question, a lot of undercover cops will buy drugs "on the street" but won't take them.

Shuji Terayama forever.