I was hospitalized, and had trouble sleeping. Doc ordered vistaril 25mg (antihistamine), which took no effect the first night. So 2nd night it was increased to 50mg. I was so exhausted, I slept just a few hours. 3rd night, it was increased to 75mg...well at that point, i had slept about 3 hours in 2 days....the 75mg kept me up another 24 hours..with super anxiety and restlessness. Same thing happened when I took Allegra and Zyrtec.