An increased risk of awareness during general anaesthesia in patients receiving tramadol has been reported. We studied whether tramadol affects the amount of propofol required for induction of anaesthesia.

In this prospective controlled study, we evaluated 46 patients, half of whom used tramadol regularly. Entropy indices, state entropy and response entropy, were used to assess the level of hypnosis. Patients were anaesthetized with a propofol infusion (1 mg kg−1 min−1) until they first became unconscious, and further until they developed a burst suppression pattern in the electroencephalogram. The doses of propofol needed to reach these end-points were recorded.

The amount (median, (range)) of propofol required for loss of consciousness was 2.0 (1.0–5.5) mg kg−1 and 2.4 (0.9–8.3) mg kg−1 (P = 0.95) in the tramadol users and controls, respectively. The amount of propofol required for burst suppression was 5.8 (3.9–12.7) mg kg−1 and 6.4 (2.9–15.1) mg kg−1 (P = 0.89) in the tramadol users and controls. There was no difference between the groups in state entropy and response entropy during different stages of induction of anaesthesia.

Tramadol did not affect the dose of propofol required to achieve loss of consciousness or burst suppression pattern in electroencephalogram during induction of general anaesthesia. However, there was a ninefold inter-individual variation in propofol dose requirement for loss of consciousness and a fivefold variation for reaching burst suppression. Due to extensive inter-individual variability, monitoring the level of hypnosis during general anaesthesia using propofol may enhance the correct dosage.