Can I take more hydroxyzine if the one dose doesn't work for me?

My sister-in-law takes hydroxyzine, but oddly she takes it for the itching related to chronic hives. One tablet was not taking care of the itching so she called her doctor and he told her to take two. I believe that's your best course of action is to call your doctor first.
Just as an aside, I was also prescribed hydroxyzine for itching for a newly developed case of dermatitis. Now I already take Klonopin for anxiety and had no idea at the time that hydroxyzine was prescribed for anxiety. When my dermatitis flares up, I try to just use the cream that was also prescribed because even though I take very little Klonopin, when I take the hydroxyzine, I sleep all day and would not be able to drive.

Before you self medicate and adjust your dose you have to inform your DR. first This is a prescription medication only, meaning you must take it as prescribed by your DR., only he/she can determine if you need a dose adjustment or not.

Usual Adult Dose for Anxiety
Oral: 50 to 100 mg orally 4 times a day
Parenteral: 50 to 100 mg IM immediately, then every 4 to 6 hours as needed

Comments:

Use should be periodically reassessed.
Treatment for longer than 4 months have not been assessed in clinical studies.

https://www.drugs.com/dosage/hydroxyzine.html#Usual_Adult_Dose_for_Anxiety

"Take hydroxyzine exactly as prescribed by your doctor. Follow all directions on your prescription label. Your doctor may occasionally change your dose. Do not use this medicine in larger or smaller amounts or for longer than recommended"

https://www.drugs.com/hydroxyzine.html