Final answer:
The Green Whistle, also known as Penthrox, is not considered illegal by the FDA. It is, however, not approved for marketing in the United States because it has not met specific safety and efficacy criteria set by the FDA as of early 2023.
Step-by-step explanation:
The Green Whistle, or Penthrox, is the brand name for methoxyflurane, which is a type of inhalational analgesic used for the relief of moderate to severe pain in a pre-hospital setting. As of my knowledge cutoff in early 2023, methoxyflurane as a pain relief option is not considered illegal by the U.S. FDA (Food and Drug Administration). However, it is worth noting that while it is not illegal, the Green Whistle has not been approved by the FDA for marketing in the United States. This means that while the drug itself is not banned, it cannot be marketed or sold in the United States until it meets the FDA's criteria for safety and efficacy.