Some Side affects that someone may experience when on nitrous oxide are nausea, headache, Vomiting, Shivering or excessive sweating and an increase in sleepiness.
These side affects don't normally happen unless the level of the nitrous becomes too high in the gas mixture or there's a sudden change in the amount of nitrous oxide that is inhaled.
I've been on nitrous oxide several times before and never experienced any of these symptoms so the nitrous oxide can be safe as long as the doctor giving the nitrous oxide knows exactly what they're doing and doesn't introduce too much nitrous oxide into your body.