Thiopentone is absolutely contraindicated in

(Porphyria): (381 -- KDT 7th edition; 536- Goodman 12th7or variegate)Thiopentone can precipitate acut intermittent or variegate porphyria in susceptible individuals therefore contraindicated. The abnormal synthesis of protoporphyrin (important in hemoglobin production) results in excess porphobilinogen. Barbiturates induce amino levulinic acid synthase, an enzyme responsible for phosphobilinogen synthesis, This leads to excessive porphobilinogen levels and can precipitate acute porphyric crises, that are manifested by severe abdominal pain, nausea, vomiting, psychiatric disorders and neurologic abnormalities