Generally nitrous oxide is regarded as being one of the safest forms of pain relief available for labour. However, as with any medications, there are a few medical conditions that can prevent the woman from using the gas safely. These include if the woman has suffered decompression sickness in the past (or 'the bends') or has had any serious lung problems such as a 'pneumothorax' (or a hole in the lung causing it to collapse).
Other health situations that would need to be discussed with the caregiver would be if the woman is a vegan or has a severe folate (folic acid) or
vitamin B12 deficiency. This is because the nitrous oxide inactivates these vitamins in the woman's system and in doing so can cause functional disturbances in her spinal cord and nervous system.
Occasionally a woman may not want to use the gas it because she feels claustrophobic with a mask on her face. In this circumstance a mouth- piece can be used. The woman may also be reluctant if she has heard negative stories about gas, or does not like the idea of it. This is her choice. The beauty of gas though is you can always stop it if you don't like it.