Opioids produce a dose related decrease in the frequency and amplitude of the EEG.
Low dose opioids show a loss of beta waves and a slowing of alpha waves.
Moderate dose opioids show diffuse theta waves and some delta waves.
High dose opioids show delta waves (high amplitude, 0-4 Hz). See Delta Waves on Wikipedia.
Unlike with volatile gases, barbituates, and propofol, complete isoelectric EEG waves cannot be obtained, even with high dose opioids