In my opinion, the notion of the white horse in any way symbolising or suggesting heroin needs to be put to rest. There is nothing in the show or the movie that indicates Palmer heroin use. What's more, it seems more likely that the horse is some sort of spirit that comes to Sarah once she's been drugged. The use of the black lodge inhabitant spotlight effect would seem to support this, in my mind.
Hard street drugs also don't seem the style of either a suburban dad or an evil spirit (BOB never gave Laura drugs...) and I may not party like I used to but- who puts heroin in people's drinks? Rohypnol or GHB, maybe.
I've posted this on the old board(s) before but still think it's a valid point: Lynch is no stranger to Jungian symbolism. A white horse in a dream can symbolize "a meeting with our feelings about death" (Tony Crisp, Dream Dictionary) Either Sarah's psychic nature is telling her that death is near or she's slipping into a death-like sleep. Trying to fight off the effects of sleeping pills and the like can cause your brain to do some weird sh*t.
The Celts (relevant to TP because of the Glastonbury thing) revered white horses for superstitious reasons (Something about the goddess Epona and a white horse.) It was a symbol of power in war for the Anglo-Saxons, especially King Alfred. Maybe the horse is a lodge spirit that protects Sarah when BOB is doing his nasty business. (We only see it twice, but it seems reasonable that BOB would drug Sarah, turn on the ceiling fan, go outside and re-enter through Laura's window.)