Promethazine has been reported to affect control of blood glucose in diabetic patients. Since promethazine is a phenothiazine, it should be used cautiously in patients with diabetes mellitus. Promethazine should be used with caution in patients with hyperthyroidism or hypertension since it can exacerbate these conditions. Promethazine should be used cautiously in patients with seizure disorder because phenothiazine can precipitate epileptiform seizures in patients with focal lesions of the cerebral cortex. Antihistamines should be used with caution in patients undertaking tasks requiring mental alertness.
Drowsiness can be severe in some patients, especially in the elderly.
Patients should exercise extreme caution until the effects are known. Alcohol can add to sedation caused by promethazine. In addition, antihistamines can mask signs of allergic reactions, thus delaying the diagnosis of a hypersensitivity reaction. Epinephrine and promethazine should be used together cautiously. Many references warn that if epinephrine is given to patients receiving chlorpromazine, epinephrine can paradoxically lower blood pressure.