The FDA puts restrictions on many types of drugs, especially ones that have addictive properties. Medications such as OxyContin, morphine, Ritalin, etc., can only be filled once, and the prescriber needs to write a new Rx each time. Drugs such as Xanax or Ambien can only be refilled for up to six months or five times (whichever is sooner), but many physicians will not prescribe more than a month at a time, because those drugs need to be well-monitored.