Is a 50-50 ratio of nitrous oxide-oxygen usually enough to make a teen patient sleepy/drowsy?

Yes: It should be more than enough since most adult patients get to that on 50/50 or less.

Dose of N2O: SWang Good day; Doses of Nitrous Oxide vary from patient to patient. Weight and age of patient are not firm barometers as to dose. The tip of a sharp explorer should not be uncomfortable to the gingival tissue-- don't need to be to sleepy/ drowsy. I try to note on record a patient's individual dose. Coupled with cell phone tunes patients are more accepting of dentistry.