Opioid drugs can all cause various impairments of body functions. The most important of these impairments are mental impairments and respiratory depression.

Mental Impairments
All opioids can impair various mental functions from drowsiness to cognitive impairments. Patients on painkillers should not drive or operate heavy equipment until they have been cleared by their doctor or health care provider. This is especially true when the medication has recently been started or the dose adjusted.

The impairments from opioids include:
Drowsiness
Confusion
Night vision
Constipation

Over time, patients on opioid medications get used to (or build up tolerance to) to most, but not all, the effects of the opioid medication. The three main effects of opioids that get patients develop tolerance to over time are respiratory depression, drowsiness and pain relief. 

If the patient takes too much or abuses the opioid, they can certainly be "high" or "stoned" but it must be emphasized that abuse is not necessary for drowsiness to occur, endangering the patient or others while driving or operating machinery.