I feel very sorry for this poor man, but equally we must all take responsibility for our actions. Plastic carrier bags are a vital feature of the tramp-chic look I've been going with this season, and to be fair, the one I'm currently carrying my laptop around in reads "plastic bags can be dangerous: to avoid suffocation keep this bag away from babies and children."

Is there anything else to worry about with hippie crack? All drugs carry risks, and it is only by clarifying those risks that we can decide how to manage them. Luckily the Medicines and Healthcare Regulatory Authority has put out a press release, which has manifested itself mostly under headlines suggesting nitrous is "no laughing matter".

The MHRA says: "Whilst the inhalation of nitrous oxide may be perceived by some as harmless activity, there are a number of health risks associated with its inhalation. The 'rush' users experience is caused by starving the brain of oxygen. This can cause the user to collapse and injure themselves when falling."

To be effective in public health policy it is generally considered that your message must be credible. I suspect most hippie crack users will already have experimented with holding their breath, and will rightly conclude that their transcendent experience on nitrous is a drug effect; and that it's an anaesthetic used in hospitals, and in childbirth, so the effect is probably not caused by starving the brain of oxygen; and that the MHRA, of all reputable bodies, is talking nonsense.

In fact, the pharmacology of nitrous oxide is fascinating, and a window on to how we deduce what drugs do in the brain. It seems to work on the NMDA neurotransmitter system, and on the opioid system. Animal models suggested that it might increase the release of opiate molecules made by your own body. To test whether opiates were responsible for the effects on humans, researchers gave nitrous to people in pain, and then gave an opiate receptor blocker, to see if the analgesic effect was reversed, or changed: the analgesic effect was, but the subjective sensations of being "high" were not, even at high doses, suggesting that this aspect of its effect must be mediated by a different neurotransmitter system. It seems likely that these effects are mediated by the NMDA system, perhaps similarly to ketamine.

Are there any real dangers with nitrous oxide? Well it's fairly safe overall, and I suppose you could risk manage the "falling over" thing by "sitting down", but it has been studied closely for a long time - on account of its widespread medical use, and the worry about chronic low dose exposure for dentists and people who work in operating theatres - and it turns out that it does have a major side effect: it selectively oxidises the folate, or vitamin B12, in your body. Vitamin B12 is needed for a process called methylation, involved in making DNA among other things, and without it you have a tough time making new cells.

There are cases in the literature of people overexposed to nitrous becoming dangerously B12 deficient, but it seems clear that the effects are reversed simply by giving high dose vitamin B12.

This may be rather trendy and "harm reduction" of me, but if I was going to put out a press release on hippie crack, I would advise against using it, but I wouldn't shoot down my credibility with primary school assertions about its mode of action on the brain. I would state the risks clearly, and if I thought the risks from, perhaps, folate deficiency were significant and worrying, I might also mention the harm reduction strategies available, and start monitoring outcomes.

· Please send your examples of bad science to bad.science@theguardian.com