Hi & Welcome. Diphenhydramine present in the OTC tonic is an old generation antihistamines which is used for allergies, hives (urticaria), insomnia, motion sickness, symptoms of common cold, etc. This is not the standard treatment for psoriasis. Diphenhydramine is known to cause sedation (drowsiness), tiredness, sleepiness, disturbed coordination, blurring of the vision, increased heart beats, dry mouth, constipation, difficulty urinating, and upset stomach. Had I been your treating physician, I would have advised the following: 1. Application of topical low potency corticosteroids containing ointments like Hydrocortisone. 2. Vitamin D analogues like Calcipotriene to be used alone or in combination with other topical medications like corticosteroids. These two are prescription medicines so you need to consult a doctor. 3. Salicylic acid ointment/cream which is available over-the-counter can be used alone or in combination with topical corticosteroids. Since, this is a chronic skin condition, you need to consult a skin specialist for proper evaluation and follow up. Hope this will help.