Truth Serum

Adventuring gear, poison (ingested)

• Properties: Poison
• Cost: 150 gp

A creature subjected to this poison must succeed on a DC 11 Constitution saving throw or become poisoned for 1 hour. The poisoned creature can’t knowingly speak a lie, as if under the effect of a zone of truth spell.

Source: Dungeon Master’s Guide p. 258. Available in the SRD