I'll skip the mini-essay about the inclusion of drugs in the Book of Vile Darkness being related to Monte Cook not being invited to the really good parties.
I have included rules for fantastic versions of two much-storied substances that I feel sure can fit into most games. It is my thought that, although these substances can add gris gris to a campaign, they should not be the focus. Thus, the rules avoid complication as well as I am able.
Addiction: Creatures that are exposed to one of these substances must make Willpower saves of the listed DC or form a habit. A creature with a habit must take a dose of his drug each day or suffer the specified drawbacks.
Each week that a character goes without his substance, he must make a Willpower save against the listed DC in order to shake his addiction. If this check fails, the character will do anything in his power that does not violate his alignment to obtain more of the substance in question. If this check succeeds, the addiction is over, though if he ever takes even a single dose of the substance again, his habit automatically reforms.
Greater Restoration or more powerful magics are required to cure addiction via magical means.
Opium (DC 18, 15gp)
This substance is usually used in a tincture or smoked via apparatuses that avoid applying direct flame to the substance. Opium products such as laudanum (opium dissolved in red wine or alcohol) are prized for their medicinal effects, and when used in conjunction with a Heal check or cure spell, allow the recipient to heal an additional +6 hp. In this case, the recipient must check for addiction.
Opium is also used for pleasure, and is renowned for the strange visions and mental activity that it causes. Each day, a creature with an opium habit may choose to memorize one of the following, which may be used as an extraordinary ability: Detect Magic, Detect Poison, Detect Secret Doors, or Detect Undead. This ability does not resemble magic, and the user may not even be aware that he is acting in an extraordinary fashion when using these abilities.
Once one has an opium habit, one must use daily or else become fatigued. If one goes more than two days without, one instead becomes exhausted. These conditions cannot be ameliorated so long as the habit lasts, except by using opium.
Absinthe (DC 14, 25gp)
The green faerie, the artist's liquor, a distillation containing wormwood and anise, among other things. This is the wizards' drug, reputed to allow men to taste souls, see the future, and hold traffic with fae creatures. When quaffed as a potion, absinthe will function as a potion with the effects of detect invisibility and detect magic at the same time, with a duration of but a single round.
A character with an absinthe habit will enjoy +1 to the DC of his spells, and enjoy a +2 bonus to Charisma-related checks with fae creatures.
When one has an absinthe habit, one must use daily or else become dazzled. If one goes without for more than a day, he instead becomes shaken. These conditions cannot be ameliorated so long as the habit lasts, except by renewed use of absinthe.