ah not just me after all 😂 as I suspected though it’s not all that well known that there are any concerns. I have made my midwife aware and she and the consultant apparently were none the wiser so asked me to share what I had seen which was this - www.gov.uk/drug-safety-update/nitrous-oxide-neurological-and-haematological-toxic-effects
it reads “Nitrous oxide is a medical gas used very widely in surgical anaesthesia. As a 50% mixture with oxygen (brands include Entonox), it is also widely used to relieve pain in childbirth and in acute trauma.
Risk with prolonged use
Nitrous oxide has a very good safety profile in normal use under the supervision of a healthcare professional. However, prolonged use may lead in rare cases to megaloblastic anaemia and neurological toxic effects (myelopathy) due to inactivation of vitamin B12.
For this reason, nitrous oxide should not be given continuously for more than 24 hours, or more frequently than every 4 days, without close clinical supervision and haematological monitoring.
Neurological toxic effects can occur without anaemia or macrocytosis, and even when B12 levels are in the normal range.
Vitamin B12 deficiency
In patients with subclinical deficiency of vitamin B12, neurological toxic effects have occurred after a single exposure to nitrous oxide during general anaesthesia. Assessment of vitamin B12 levels should be considered before nitrous oxide anaesthesia in patients with risk factors for deficiency of this vitamin. Individuals at risk include elderly people, those who have a poor or vegetarian diet, and those with a history of anaemia.”
Theres also this - pubmed.ncbi.nlm.nih.gov/8250714/
which reads “Vitamin B12 (cyanocobalamin) is an integral component of two biochemical reactions in man: the conversion of L-methylmalonyl coenzyme A into succinyl coenzyme A and the formation of methionine by methylation of homocysteine. The transmethylation reaction is essential to DNA synthesis and to the maintenance of the myelin sheath by the methylation of myelin basic protein. Active vitamin B12 contains cobalt in its reduced form (Co+). Nitrous oxide produces irreversible oxidation to the Co++ and Co forms that renders vitamin B12 inactive. Five cases (four from the literature and one new case) are presented in which patients unsuspected of having vitamin B12 deficiency developed subacute combined degeneration of the spinal cord following nitrous oxide anesthesia. Patients with vitamin B12 deficiency are exceedingly sensitive to neurologic deterioration following nitrous oxide anesthesia. If unrecognised, the neurologic deterioration becomes irreversible and may result in death.”
I’m not sure I understood all that but it does sound like if you’re monitored it’s perhaps ok. That the levels drop rather than anything more untoward.