Fentanyl gas was used by authorities in the former Soviet Union to break up a hostage situation and resulted in many deaths of the hostages they were trying to save because they stopped breathing and,or had heart stoppage.. It's not something anyone not in a great deal of pain would be given under any circumstances and I don't think anyone could take it and drive anything legally, although I have heard of people taking Morphine Sulfate continuous dosage tablets (MSContin) for severe pain and still driving a car. OxyContin is also used about the same way.
The patient becomes accustomed to the drug and can function better with it than without it because of severe pain.