Matter of duration . One can achieve the same effect with either one. It's just a matter of dosage or amount. What's different with the two is half-life or length of duration of effect.
Benzodiazepines. Benzodiazepines are a class of medications used primarily now for short-term treatment of anxiety disorders. Their use should be limited to about 4-6 for most patients, although some do well with chronic use. Lorazepam is actually less potent than alprazolam. 1 mg of alprazolam is generally about equivalent to 2 mg of lorazepam. Physicians adjust doses to the desired effect.