SR or XR or similar sustained release tablets are, as you say, designed to slow down the release of the drug from the tablet, so as to produce more steady blood levels of the drug through the day ; and can have the advantage of convenience in that you may be able to take them once a day rather than two or three times a day.
In the standard form of drugs, they tend to dissolve more rapidly, giving a more rapid rise in blood levels of the drug, to an initially higher level, which then drops gradually until rising again when the next dose is taken.
The difference may or may not matter much. There may be an inconvenience of some side-effects being experienced during the short time that blood levels are higher, with the ordinary pill : but many people don't experience this nor are bothered by it. Where the SR version is more expensive than the ordinary version of a drug, the extra cost may be unwelcome.
When a new drug is developed there are very high costs in research to prove that it is effective and safe, so companies are given an initial period when they have an exclusive right to market the new drug ; and after some years this exclusivity lapses and other companies are also allowed to make and sell it, usually at lower, generic, prices.
I am sometimes suspicious that the benefits of slow release versions may be exaggerated, because in many countries, when a company's original exclusive patent is due to run out, they can get an extension of time by producing a revised version, not of the chemical drug itself, but of the way it is presented : in an SR / XR version, even if that isn't needed or valuable, or some other such variant, and it would be some years before competitors would be able to produce their own versions of that variant.
Anyhow, back to your specific question.
I'm hearing a number of reports of some drugs being out of stock ( which ideally should not happen ) and about what alternatives are offered. In your situation, it is useful to avoid a sudden and lasting drop in blood levels of the drug, such as would happen if you took no version of Xanax / Alprazolam at all. But it is not essential to maintain very steady blood levels so long as they remain within the usual tolerable levels.
You should discuss this situation with your prescribing doctor, and/or with the dispensing pharmacist. Your best bet would be to take the standard version you've been given, at the properly prescribed and recommended dose ( you don't mention what advice you were given ) such as 0.5 mg twice a day, morning and evening, though I'd prefer a somewhat lower dose.
It is usually a bad idea unless instructed by the doctor or pharmacist, to start cutting up tablets. This is an especially bad idea with SR type products, as this may cut through a specially designed and prepared outer coating which controls the release or the drug, exposing the core and causing a more sudden release than intended.
Maybe your doctor is unaware that the SR version is temporarily unavailable, or he would probably have advised you on just what to do with the standard tablets ; your pharmacist, who seems to have made the substitution, ought to have given you clear instructions on how to manage the change.
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