Many treatments. Poison ivy, oak and sumac rashes are an allergic response to oils produced by these plants. If you suspect you have been exposed wash your skin well to remove the oil. If you discover the itchy red rash instead, you will need to treat the symptoms. Anti-tich measures : baking soda, diphendyramine, and avoid overheating. Hydrocoritsone cream and domeboro soaks help too. See your doctor if severe.
Steroids. Technically not an allergy but a contact dermatitis, poison ivy, oak and sumak can be a misery. If widespread, see your doctor right away and save yourself the grief of otc treatments not being effective enough. Even if not widespread, if you aren't getting adequate relief, see you doctor for a prescription for a steroid cream for much quicker relief.
Hold the baking soda. If the area of affected skin is small use otc 1% Hydrocortisone cream 2-3x/day until skin is clear for 3 days. If the area is large contact your md to get oral corticosteroids.
Rhus -Posison Ivy. Clorox water before any significant blisters. Calomine or calodrl, aloe vera leaves or baking soda paste to palliate. Hydrocortisone creame otc. For more severe cases see your doc.