Pheromone is a bright red sparkling liquid that stains creatures with the charm curse, making them switch allegiance to the side of the player. It will stain the player and cause the same particle effects as on enemies, but is otherwise harmless. It can be found in flasks just as many other Magical Liquids, but it can also can be created by mixing Flummoxium and Berserkium.
Pheromone has a natural propensity to form invisible, flying creatures of largely unknown nature which are naturally friendly to the player. This effect can only be observed if a pool of pheromone is sufficiently deep (somewhere around 5-10 pixels in depth).
These creatures are largely unnoticeable, but will quickly pick up dropped wands and attack any nearby enemies. They have a tendency to wander off, unlike most charmed enemies, and seem to be untouched by Pheromone's usual stain. While not holding a wand, they have an innate short-range projectile attack that appears similar to a short-range Firebomb, although the color and particle effects are noticeably different.
These creatures will drop 10 when killed (20 if the death was an accident), and as such pools of pheromone can be exploited to gain some quick gold if there is an Emerald Tablet, Runestone, or other physics object available to quickly dispatch the ghosts. Throwing tablets or other physics objects across the top of a pool of Pheromone will generally net a decent yield of gold, seemingly from thin air.
What charm actually seems to do is set a creature to belong to the "player faction", overriding any previous allegiance. As mentioned, this does nothing when staining the player, but there is an exception to this rule. A fringe case where if you have the Plague Rats perk and then get stained with Pheromone your own rats will actually attack you. This is because when you take that perk your faction is set to the rats, and the pheromone overrides it.