The tempo and the dynamics (or the volume level) of the song track Reed's lyrics that make it sound almost as if we're listening to the internal soundtrack of a heroin bender in real time. When Reed sings "When I put a spike into my vein," for example, Moe Tucker's drumbeat doubles in speed and the guitars begin strumming rather than plucking the chords. Tucker's rhythm, as simple as it is, sounds like a heartbeat, speeding up with the injection of the drug and then coming down when Reed sings, "Well I guess that I just don't know."
Meanwhile, John Cale plays one steady note for nearly the entire duration of the song. This is what is called a "drone." It's exactly what it sounds like, a sustained note that lasts basically forever. Cale's use of the drone in "Heroin" was influential in bringing the drone to rock (or "post-rock," rather). The drone rises in volume with the rest of the instruments during the song's "highs" but only gently, like the ebb and flow of a nice calm breeze. The drone gives the song a surreal kind of half-conscious peace.
The feeling of stability that Cale's drone gives to the song makes the atonal collapse that defines the end of the song all the more powerful. The two-minute screeching of Cale's viola might be the real "high" of the user, but that doesn't change how utterly unpleasant it is to listen to. Many have called "Heroin" a decadent song about drug use, but there's nothing decadent about the feeling of disintegration that the viola creates in the song's last minutes. The viola becomes a representative of self-destruction; this is, in short, the miserable sound of addiction.