Oral antihistamine pills, including diphenhydramine (Benadryl) and loratadine (Claritin) can relieve itchy skin caused by a poison ivy rash. Taking oral antihistamine pills at night can be especially helpful if your itchy rash is making it difficult for you to fall or stay asleep. But keep in mind that you should not apply topical antihistamines directly to your skin, as this can worsen the rash and itch.