Promethazine is a phenothiazine derivative that competitively blocks histamine H1 receptors without blocking the secretion of histamine. It also is a very weak dopamine antagonist
Excessive sedation may occur when promethazine is combined with other medications that depress the central nervous system (brain) and cause sedation. Such drugs include ethanol, barbiturates, anti-anxiety medications, sedatives, other phenothiazines, and narcotic pain medications.
A complex called neuroleptic malignant syndrome (NMS) can occur in patients receiving phenothiazines. NMS consists of high body temperature, severe EPS, changes in consciousness and mental status, and increased heart rate with low or high blood pressure. NMS occurs more frequently in young men and in persons who are dehydrated.
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