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"In 1961, the British Oxygen Company introduced a single-tank delivery system marketed under the trade name Entonox® (The Linde Group, Munich, Germany) that continues to be used today in the United Kingdom.10 Entonox has never been approved by the US Food and Drug Administration (FDA) for use in the United States; the only delivery apparatus used for this purpose in the United States is Nitronox® (Porter Instrument Division, Parker Hannifin, Hatfield, PA). This delivery system combines 50% N2O and 50% oxygen in a set concentration that cannot be altered, which differs from the apparatus used primarily in dental offices. The equipment used in dental practices allows for variable concentrations of N2O delivery, and is not intended to be used for patient-controlled N2O delivery. Although other approaches (such as continuous administration of concentrations, < 50% N2O in oxygen and intermittent administration of higher concentrations) have been employed and may offer modest improvements in pain relief, the intermittent use of 50% N2O in oxygen probably optimizes patient safety and has thus been most widely adopted."
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3594866/
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"In 1961, the British Oxygen Company introduced a single-tank delivery system marketed under the trade name Entonox® (The Linde Group, Munich, Germany) that continues to be used today in the United Kingdom.10 Entonox has never been approved by the US Food and Drug Administration (FDA) for use in the United States; the only delivery apparatus used for this purpose in the United States is Nitronox® (Porter Instrument Division, Parker Hannifin, Hatfield, PA). This delivery system combines 50% N2O and 50% oxygen in a set concentration that cannot be altered, which differs from the apparatus used primarily in dental offices. The equipment used in dental practices allows for variable concentrations of N2O delivery, and is not intended to be used for patient-controlled N2O delivery. Although other approaches (such as continuous administration of concentrations, < 50% N2O in oxygen and intermittent administration of higher concentrations) have been employed and may offer modest improvements in pain relief, the intermittent use of 50% N2O in oxygen probably optimizes patient safety and has thus been most widely adopted."
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3594866/