Cordial of the Dryad
 
Polyhedron - 1st Ed. AD&D - Polyhedron #19
Second Runner-Up: Cordial of the Dryad (for the AD&D game) 
Rating: Usefulness #3; Originality #1; Rules Adherence #3

This is a clear liquid with emerald or amber tint, a rich, sweet
taste, and a distinctive scent (as that of a forest after a summer
rain). It is made of the distilled essence of a dryad's tree. This
process is known only to certain druids. When consumed by a
dryad, the cordial enables the creature to survive comfortably for
24 hours while away from her tree at distances greater than the
usual 36" limit (almost always to accompany and serve a druid).
Special note: potion miscibility does not apply to this item.

If consumed by any other creature, the cordial's effects are
based on the user's sex, as follows.

Female: No effect occurs for three rounds. In the fourth round,
the user falls asleep. This "beauty sleep" lasts for 2-5 hours,
during which all imperfections, blemishes, etc. disappear. The
character gains one point of Charisma <> and becomes magically
attractive to males of the same race (only), sometimes to the point
of causing arguments among them. (Optional: Also gain four
points of Comeliness). With practice, the user can control this as a
limited Charm Person effect (+2 bonus to victim's saving throw;
note that the effect still applies only to males of the same race).
The potion's effects last for 1-4 weeks, after which all ability scores
return to normal except for the permanent gain of the point of
Charisma. (Optional: plus two points of Comeliness).

Male: No effect occurs for three ruonds. In the fourth round
the user becomes weak and light-headed, and falls asleep. This
"transition sleep" lasts for 3-12 hours, during which the victim
becomes a female and loses two points of strength. No special
powers are gained. The change lasts 2-5 weeks, after which the
character returns to normal in all respects except for the permanent
loss of one point of strength.

Multiple use: The second cordial consumed by any one person
has double the given duration; the third has double that duration,
and so forth. Additional effects are as follow:

Female: The user may actually become a dryad: 10% chance
for the second cordial, 25% for the third, 50% for the fourth, and
100% for the fifth. If this change occurs, she is immediately
bound to the nearest suitable tree.

Male: The effects may be permanent and unremovable: 20%
chance for the second cordial, 50% for the third, and 100% for the
fourth.