I'm not getting any answers here. My friend fainted after being put on nitrous oxide in the dental chair for 20 seconds (under 50 percent nitrous oxide / 50 percent oxygen). Can someone explain to me why or if this has something to do with the fact he was on claritin for a stuffy nose? I've heard dentists aren't supposed to use nitrous if someone has a stuffy nose. Is this for the patient's health, or just because it's easier to get nitrous in with a clear nose? Also, why did he experience immediate tremors (convulsive-like movements)?