Can Alprazolam be used on an as-needed basis?

Yes. Absolutely. Take it only when needed but do not exceed maximum daily dose. If your mom doesnt need it at all some days no need to take it but if on others she needs it for every dose thats ok too. The Alprazolam will actually be more effective when taken this way because her body won't build up a tolerance as quickly to it. As far as the glaucoma goes you can always call her Dr & make absolutely sure theyre aware of it & wants to continue with this med but a lot of times a Dr weighs the risks of taking the medication vs. the risks of not having it. If the risk to her glaucoma with it is minimal then the pros out weigh the cons.

Yes, that is exactly how it should be used. It is not to be taken regularly. I put it in a category I call my "rescue" meds. You start to have an anxiety attack, take it while thinking positive thoughts as it dissolves, and hopefully breathing and heart rate will slow down, as will the stinkin' thinkin'. ;-)

Now... glaucoma. Benzodiazepines... ANY of them are contraindicated in patients with acute angle-closure glaucoma OR untreated open-angle glaucoma. Benzos don't have anticholinergic (a substance that blocks the neurotransmitter acetylcholine in the central and peripheral nervous system) activity, but (only rarely, yet it can happen) has been tied to increased intraocular pressure.