The effects of drugs on the EEG vary and are often dose dependent. In general, drugs effects on the EEG are fairly predictable and include: no effect, accentuation of beta activity, background slowing with decreased amplitude and/or frequency of the alpha rhythm, intermixed theta and/or delta activity, decreased seizure activity, and lowering of the seizure threshold with increased spike and wave discharges. In severe overdose cases, the EEG may demonstrate alpha/theta coma, burst suppression, or even a "flat" EEG pattern.
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Walt Banoczi (2005) studied this question.
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