Barbiturates produce all of the following effects EXCEPT:

Correct answer: A. Analgesia

• A. CORRECT as the EXCEPTION. Barbiturates have NO analgesic activity. More than that, at low doses they are ANTI-analgesic — they can heighten the perception of pain and increase restlessness in a patient who is already hurting. This is the reason a barbiturate must never be used alone to manage a painful procedure; local anaesthesia or a separate analgesic is mandatory.
• B. Sedation is produced at lower doses through enhancement of GABA-A receptor activity — barbiturates increase the DURATION of chloride channel opening, whereas benzodiazepines increase the FREQUENCY.
• C. Hypnosis (sleep induction) is a well-established barbiturate effect and the basis of their historical use as sleeping agents.
• D. At high doses barbiturates do produce general anaesthesia; thiopental was the classic induction agent for exactly this reason.

Note: The same 'no analgesia' warning applies to benzodiazepines and to nitrous oxide at sedative concentrations. Note also the crucial pharmacological difference in safety: barbiturates can open the chloride channel directly at high doses, which is why they have no ceiling effect and can cause fatal respiratory depression — and why flumazenil, which reverses benzodiazepines, does nothing for a barbiturate overdose.