Who should not have nitrous

Nitrous oxide is among the safest drugs in dentistry, and the list of people who should not have it is longer than the list in common circulation. Two mechanisms do almost all of the harm, and they have nothing to do with each other: nitrous moves into any closed gas-filled space far faster than nitrogen moves out, so it expands or pressurises that space, and it irreversibly oxidises vitamin B12, shutting down methionine synthase — a laboratory event after one short sedation in a patient with normal stores, and a demyelinating one with the repeated exposure of recreational use. What a handful of ordinary clinical exposures does to a patient with marginal stores is the part the evidence does not settle. The first mechanism makes recent eye surgery with a gas bubble, pneumothorax and bowel obstruction real contraindications, with recent middle-ear surgery a caution whose magnitude is genuinely debated; the second makes the patient with undiagnosed B12 deficiency, and the young patient using canisters recreationally, the ones a standard health history is not built to find. The most common reason nitrous fails is neither of those mechanisms, and it outnumbers every pharmacological contraindication put together: the patient cannot breathe through their nose. And the blood pressure that falls when you start it is the anxiety leaving, not a drug lowering the pressure, so do not treat it as an antihypertensive or be reassured by the reading.