The medications typically in short supply are generally low cost drugs such as morphine sulfate, zofran, compazine, reglan, ativan and valium.. The shortages often involve injectable drugs which are more technically difficult to manufacture than oral medications.

FORBES: Drug Shortages Compromise Emergency Medical Care

The FDA's warning letters notified the companies they may be subject to legal action if they do not stop manufacturing and distributing "prescription unapproved products" that include high-concentrate morphine sulfate oral solutions and immediate-release tablets containing morphine sulfate, hydromorphone or oxycodone.

CNN: FDA warns makers of unapproved narcotics