What was Frank Booth's drug?

In several scenes throughout the film, Frank Booth (played by Dennis Hopper) uses a mask to breathe in gas from a tank, and what the gas actually was has caused considerable debate over the years. In the script written by David Lynch, it is specified as being helium, to raise Frank's voice to a higher pitched tone. On the DVD, Hopper explains that when he filmed Blue Velvet, he had just gone through detox and was clean for the first time in several years. He brought with him considerable knowledge of various narcotics, hallucinogens, etc, and told Lynch that he didn't feel helium would be right. Rather, he said, he believed that Frank would have a tank of amyl nitrate (also known as "poppers"), a drug used medically by heart patients to regulate their heartbeat, and recreationally in altered doses to relax the muscles and induce a state of euphoria. Hopper said that he had experimented with the drug, and that during his highs he experienced visual hallucinations of glowing objects in front of him. Lynch liked the idea and incorporated it into the film; hence Frank's "grabbing" motions in some scenes after he's been inhaling.

Hopper would later go on to say that he wished that he hadn't convinced Lynch not to use helium, as it only occurred to him after filming how unsettling it would be for a maniac to be inhaling it and then threatening people with a high pitched voice.