Xanax 0.25 mg tablet is the starting dosage in adults these tablets minimum of three times the doctor-advised dose depending on how you respond to starting doses. Xanax 0.25 mg tablet is used to treat anxiety disorder and anxiety caused by depression. Xanax will cause sedation and drowsiness as some side effects but is not insomnia. A person may take it before bed to help them fall asleep, and relax, and side effects within about eight to twelve hours. Xanax can be taken about an hour before a performance event. Xanax increases the level of a brain chemical called GABA.