@McBain
Lol pictures pls. I just popped 3 hair tonics, no idea if they stack.
Apparently they do (my dude's looking quite feral...no zztop yet though):
Drinking one doubles the Hair Growth rate of the player. The effect stacks - drinking 2 speeds up the rate to 4, drinking 3 increases it to 8 etc - and ends once the next stage of growth is achieved.
The player's hair and facial hair will only naturally grow to a length of 7, but hair tonic can increase the beard's growth up to a 10. Levels 8 and 9 take roughly two weeks to reach with one dose of tonic each, while reaching a level 10 beard takes roughly 3 weeks of in-game time.
Until beard stage 5, consuming more than 3 at a time is considered wasteful, as the time required to progress to the next stage is relatively short. However, from this point onward the growth progress normally takes up several in-game weeks, so taking more than 3 can speed up the progress considerably.
The player needs to trigger a "screen fade" to allow the hair/beard progress to the next stage; they do not grow in real time, and the progress will happen "off-screen", such as while sleeping or fast traveling.