The maximum safe dose of Phenergan (Promethazine) is as follows- Oral tablets 50 mg as a single dose every 12 hours Injection 50 mg IM , every 12 hours Reference British National Formulary-2008 This maximum dose is used under special circumstances. The dose commonly prescribed is Oral tablets 25 mg 12 hrly Injections 25 mg IM 12 hourly
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Tablets--one 5-mg or 10-mg tablet three to four times a dayExtended-release capsules--one 15-mg capsule first thing in the morning or one 10-mg capsule every 12 hours Suppository--25 mg, twice a day
50mg is the largest dosage but I would only take that if you are on the larger side. 25mg in the normal dosage.