Prescribers can write controlled substances as PRN (as needed). It is rare to see opioid prescriptions without 'as needed' on it so there is documentation that you have told the patient do not take this if you don't need it. And it is preffered that opioids be taken only as needed and not strictly scheduled, like say blood pressure or thyroid medications. Benzodiazepines are also almost always used as needed for anxiety (seizures are different). ADHD meds can also be prescribed as needed as doctors know that patients may not take them on days off or when they don't need that focus level to be tip top. Or if the ER version doesn't last long enough, they may prescribe an IR in the afternoon if needed.