In Pulp Fiction some of the protagonists are very much into using heroin. I would think cocaine was the drug of choice by then, especially among the well-to-do in L.A. Was heroin in fact (still/again) a party drug back then or was heroin just a plot device in this movie?
This is a difficult question to answer because it's so close to the 20-year barrier. In fact, I only have one plausibly valid source; keep in mind that it was published in 1997. "What America’s Users Spend on Illegal Drugs, 1988-1995", put out by The Office of National Drug Control Policy. They have it handily in PDF form here: https://www.ncjrs.gov/ondcppubs/publications/pdf/retail.pdf Anyway, that stands habitual cocaine use at 3.0-3.6 million users and habitual heroin use at 690,000 - 880,000 users.
Another source that is definitely illegal is Clinton decrying "heroin chic" on 1997-05-21: http://www.nytimes.com/1997/05/22/us/clinton-calls-fashion-ads-heroin-chic-deplorable.html. The significance being that Pulp Fiction was one of multiple references to heroin usage in popular culture and mainstream media, at least by 1997, according to Clinton.
However, I don't feel confident answering your question directly.