I've always thought they were "Poppers" (Amyl nitrite) in capsule-like vials that he breaks open and inhales through the nose, rather than swallows like pills. It gives you an intense, temporary "rush." I've been out of the drug culture for decades, so I don't know what specific kind he was taking, but obviously it had some nasty effects.Interesting. In the DVD documentary of Blue Velvet, I believe that they mention that amyl nitrate was the gas that Dennis Hopper's character inhales.