Merasha

Merasha is a drug that severely disables Deryni, with nausea, dizziness, blackouts, and disorienting effects on the brain that prevent the drugged person from concentrating and making use of their Deryni powers. In ordinary humans, it only produces a mild drowsiness. It is a tawny colored liquid.
The Codex Derynianus states that it was first obtained by boiling down the stalks of the merash bush in the Anvil of the Lord, and then skimming off the residue. (Alaric Morgan Duke of Corwyn mistakenly believed the drug to have been discovered and developed by Charissa, Pretender of Gwynedd.)
It first came into extensive use under the Regency of King Alroy, when it was employed both to detect and to disable the regime's Deryni enemies. Originally administered orally, merasha could also be introduced directly into the bloodstream through the so-called "Deryni prickers" as well as placing it on knives, swords and arrow heads.