Otosil 25 mg is a prescription antihistamine tablet containing Promethazine Hydrochloride, manufactured by Opsonin Pharma Ltd. in Bangladesh. It is used to treat various allergic conditions, nausea, vomiting, motion sickness, and insomnia.Otosil 25 mg is a prescription antihistamine tablet containing Promethazine Hydrochloride, manufactured by Opsonin Pharma Ltd. in Bangladesh. It is used to treat various allergic conditions, nausea, vomiting, motion sickness, and insomnia.