You'll find fluoride in tea, beer and fish, which might sound like a balanced diet to you. But this week Alan Johnson announced a new push for putting it in our drinking water, with some very grand promises - and in the face of serious opposition.

General Jack D Ripper developed his theories about environmental poisoning and bodily fluids when he experienced a pervasive sense of emptiness during the physical act of love. He instantly identified the cause, as documented in Dr Strangelove: "Do you realise that in addition to fluoridating water, there are studies under way to fluoridate salt, flour, fruit juices, soup, sugar, milk, ice cream? Ice cream, Mandrake? Children's ice cream! You know when fluoridation began? 1946, Mandrake. How does that coincide with your post-war Commie conspiracy, huh?

"It's obvious, isn't it? A foreign substance is introduced into our precious bodily fluids without the knowledge of the individual, and certainly without any choice. That's the way your hard-core Commie works."

Bill Etherington MP calls fluoride "poison". Nazis supposedly used it to subdue people in concentration camps. According to a former Guardian alternative health columnist, fluoride is "in the same league as lead and arsenic".

The reality is that anyone making any confident statement on fluoride speaks way beyond the evidence. In 1999 the Department of Health commissioned the centre for reviews and dissemination at York University to do a systematic review of fluoridation and its effects on dental health. Little new work has been done since. In the review, 3,200 research papers, mostly of very poor quality, were unearthed. The ones that met the minimum quality threshold suggested there was vaguely, possibly, around a 15% increase in the number of children without dental caries in areas with fluoridated water, but the studies generally couldn't exclude other explanations for the variance. Of course, the big idea with fluoride in water is that it can reduce social inequalities in dental health since everyone drinks it. But there isn't much evidence on that either.

So when the British Dental Association says there is "overwhelming evidence" that adding fluoride to water helps fight tooth decay, it is in danger of stepping into line with Ripper. And when Johnson says fluoridation is an effective, relatively easy way to help address health inequalities, he is really just pushing an old-fashioned line which says complex social problems can be addressed with £50m worth of atoms.

But since I'm in the mood for scaremongering, let's not forget the potential harm. A study from Taiwan found a high incidence of bladder cancer in women from areas where the natural fluoride content in water was high. It might have been a chance finding; but it could be real.

The problem is one of small effect sizes. Fluoride and bladder cancer would be a pig to research as the effect size is small, the exposure runs over half a century, and the outcome - bladder cancer - takes a lifetime to reveal itself. Welcome to the finer details behind "more research is needed". And the numbers can get very scary, very quickly: in the UK a 10% increase in risk would give you 1,000 extra new cases of bladder cancer a year. Fear. Actually, I enjoyed that. Maybe I should move to the Mail.