Hydroxyzine has the greatest impact 30 to 60 minutes after taking it. It has a half-life of 4 to 6 hours. This means that it remains in your system for four to six hours after you stop taking it. Hydroxyzine works by blocking certain chemicals in the brain that are involved in feelings of fear and anxiety. It is used to treat anxiety disorders such as generalized anxiety disorder, obsessive-compulsive disorder, and panic attacks. It may also be used to relieve the symptoms of stress, depression, and other mood problems. Hydroxyzine comes in tablets that contain between 2.5 and 12.5 mg of the drug. The usual dose for adults is between 10 milligrams (mg) and 50 mg at one time. Higher doses may be needed to treat severe anxiety or repeated episodes of panic attack. The tablet should be taken with food or antacid as it can cause diarrhea if taken without food or acid in the stomach. It takes about two weeks to build up a resistance to hydroxyzine. When this happens, the drug no longer works effectively. Therefore, continuous use of hydroxyzine requires maintenance doses to keep effective levels in the body. Hydroxyzine is safe when used as prescribed for anxiety disorders.