Can you let me know is inhaling nitrous oxide harmful?

Monitoring : If you are properly monitored by a qualified health care professional its safe, otherwise it can cause hypoxia (low oxygen levels) especially at high concentrations...

Yes: Nitrous oxide (n2o) is an anesthetic gas administered during general anesthesia and in dental practice. It is an odorless, colorless gas that is supplied in pressurized tanks. It was the first anesthetic gas discovered in the 1840s in the us. With long term exposure n2o can cause bone marrow and nervous system damage associated with B12 deficiency. As such, it is no longer widely used.

No & Yes: In the hands of an experienced anesthesia care & dental providers, n2o can be one, of several, excellent anesthetic choices for certain kinds of surgery but only in patients with low risk for n2os side effects which include cardiac dysfunction, high lung resistance, pregnant state, following eye surgery, etc. It is inappropriate & unsafe to use n2o recreationally.