drug related trivia (from wiki)
Throughout the film, Frank Booth uses a mask to breathe a gas from a tank. The identity of this gas is a subject of controversy. Lynch's script specified helium, to raise Frank's voice and have it resemble that of an infant. However, during filming, Hopper, an experienced drug user, claimed to have insight into Frank's choice of drug and that helium was inappropriate:
"...I'm thankful to Dennis," Lynch said, "because up until the last minute it was gonna be helium — to make the difference between 'Daddy' and the baby that much more. But I didn't want it to be funny. So helium went out the window and became just a gas. Then, in the first rehearsal, Dennis said, 'David, I know what's in these different canisters.' And I said, 'Thank God, Dennis, that you know that!' And he named all the gases."
In a documentary on the DVD version of the film, Hopper claims that the drug was amyl nitrite an angina medication that was first used recreationally as an inhalant in the disco club scene. However, amyl nitrite is a yellow-gold liquid which is inhaled as it evaporates, not a gas that comes in a canister. Frank's drug is more likely nitrous oxide, commonly known as laughing gas. It is used legitimately for dental anesthesia and making whipped cream in aerosol containers. It is also used as a recreational drug, because in small doses,it produces a brief but intense high. In larger doses, nitrous oxide can cause auditory and visual hallucinations.