Final answer:
Out of the given drug options, acetaminophen is the safest to administer to a patient already on oxycodone because it is a non-opioid pain reliever that does not have the abuse potential or risk of respiratory depression that opioids carry.
Step-by-step explanation:
When considering a second drug to be administered to a patient already on oxycodone, one must carefully choose an option that is safe and does not increase the risk for serious side effects or overdose. Oxycodone is a potent opioid used to relieve moderate to severe pain, and it has a high potential for abuse. Among the listed options, acetaminophen is a non-opioid pain reliever which is often combined with opioids like codeine to treat mild to moderate pain, and it does not have the same abuse potential or respiratory depression risk associated with opioids.
Fentanyl and morphine are both very potent opioids and would increase the risk of opioid overdose when combined with another opioid like oxycodone. Codeine, while it is an opioid, it is of relatively low potency and is often prescribed for minor pain. However, combining multiple opioids is generally not advisable given the increased risk of side effects and overdose. Therefore, out of the given options, acetaminophen would be the safest drug to administer to a patient already on oxycodone.