Any time someone must take medication(s) for a long period of time, they're likely to encounter some kind of problem getting the correct quantity every month, due to a shortage of the drug from its suppliers. The Food & Drug Administration (FDA) regularly publishes accurate data on the current state of drug shortages in the US; other sites also perform this function. The best site to check regularly for shortage information is the FDA's Current Drug Shortages Index (this is the same link as #2 below).
Back-ordered Drug Information from the Veterinary Anesthesia & Analgesia Support Group, a very complete and detailed listing of drug shortages. Limited to veterinary drugs, but there's so much overlap that this is a most useful list.
Current Drug Shortages also from the FDA, this constantly-updated page lists the drugs in short supply.
Drug Safety and Availability from the FDA, "information for consumers and health professionals on new drug warnings and other safety information, drug label changes, and shortages of medically necessary drug products."
Drug Shortages "FDA takes great efforts, within its legal authority, to address and prevent drug shortages, which can occur for many reasons, including manufacturing and quality problems, delays, and discontinuations."
Drug Shortage Resources from the American Society of Health-System Pharmacists (ASHSP), an excellent collection of articles on current drug shortages, usually from their journal (Am J Health-Syst Pharm.) but from many others as well.
Drug Shortages: A Problem for Everyone a great presentation (less than eight minutes) by Dr. John Maris, Chief of Oncology at the Children's Hospital of Philadelphia, who had just returned from testifying before Congress about the nationwide drug shortages.
Drug Shortages: Current Drugs also from the ASHSP, a series of drug shortage bulletins, sortable by drug name or revision date. This is the most complete listing of current drug shortages I have ever seen! Bulletins are posted as soon as they're received, and there are a lot. From 2/07-2/10/2012, the daily bulletin total was 12, 27, 8, and 12, respectively.
Drug Shortages Resource Center also from the ASHSP, "the first stop for ASHP information and resources on drug product shortages and management." There are three very useful lists of drug bulletins:
Executive Order 13588 -- Reducing Prescription Drug Shortages the full text of President Obama's Executive Order, designed to address the serious problem of drug shortages in the US.
Funding Bill May Address Drug Shortages: Aides an excellent summary of the several bills now before Congress, pushed ahead by President Obama, that should address the shortage problem very effectively. Now if we can just get Congress to pass them, without a lot of games and posturing.