Dreams of poison often fall into general categories of power, suicide, or revenge. Poison is a powerful image for us, as from childhood we were all taught the dangers of it.
Power dreams often involve drinking poison without consequence. Many times, the dream world provides the ability to act without concern for outcomes and to assert free will over social constraints. The poison dream can represent this, depending on what poison was consumed, under what initiative, and who watched the deed.
Suicide-by-poisoning dreams often have little to do with suicide, but much to do with peer-pressure, difficult work relationships, or a lack of personal power. In dreams of this nature, the dream may be teaching you that a certain part of your life is self-destructive.
Finally, the dream of poisoning as an act of revenge may reveal latent or unexpressed anger towards the object of your revenge. Poison is a satisfying way to commit a crime because we often associate it with painful death and sneakiness.