I've touched on the idea of homeopathy several times in the past, but I thought it might be a good idea to go through a quick description of what it is and why it doesn't work.
What is it?
Homeopathy is an alternative theraputic modality. It was invented by a man named Samuel Hahnemann in the late 18th century, when mainstream medical technology still included things like leeches, bloodletting and trepanning. Hygiene hadn't been invented yet, and imbalances in the Four Humours were thought to be the cause of disease.
At that time, a patient undergoing medical treatment was more likely to die from secondary infections and and side effects from treatment than to recover, so Hahnemann created what he believed to be a gentler alternative: Homeopathy.
Homeopathy is based on two main principles: Similia Similubus Curentur and Dilution. Let's look at each of those separately.
Like Cures Like
Similia Similubus Curentur (or Like Cures Like) is based on a "fight fire with fire" approach to handling disease. An idea not all that dissimilar to the use of vaccinations in modern medicine, but in a much more primitive form. It's primitive because Homeopathy employs this approach reactively and at a symptomatic level, rather than pre-emptively dealing with the cause.
The process begins with a "Proving", in which a healthy patient is given a dose of a particular substance, and is observed to see what symptoms it induces. Once the symptoms have been documented, that substance is then used to treat sick patients who demonstrate that same cluster of symptoms.
For example: caffeine would inhibit the ability to sleep in a healthy patient. So the Homeopath will treat an insomniac (who is already unable to sleep) with caffeine. Or the Homeopath might treat a patient suffering from a fever with chilli, and so on.
On the surface this seems counter intuitive. And that's exactly what it is. There is simply no known mechanism through which this might work. It's simply a ridiculous idea, and Homeopaths would need to present some pretty extraordinary evidence to prove that it works. The problem is that that evidence simply isn't forthcoming.
Somehow, Hahnemann got into his head a very strange idea: that the more you dilute a substance, the more potent it becomes.
When we talk about dilutions in the context of Homeopathy, we're not talking about dissolving a teaspoon of sugar in a glass of water. That wouldn't be potent enough! We're talking about dilutions so great that we need superscripts to be able to write the numbers!
For instance, it's not uncommon for us to see Homeopathic remedies available at 200C. This is Homeopathic code for a dilution of 1 part active ingredient to 10^400 parts of water. (10^400 is a number with a 1 in the front and 400 zeroes behind it. I'm not going to type all that out. As far as I know, that number doesn't have a name.)
The problem comes in with a well-understood concept in both chemistry and physics called Avagadro's Constant. Avagadro's Constant tells us that once you get to a solution of 1:10^23 , you only have one molecule of the active ingredient left in the mixture.
So as soon as you get to a solution of 1:10^24, you have a one in ten chance of there being a single molecule in the mixture. 1:10^25 takes that to one in a hundred, and so on. Once you get to 1:10^400, the chances of there being even a single molecule of the active ingredient in the preparation are far too small to even consider.
So what are they selling? Pure water? Yup. Sometimes they'll dilute the substance in something other than water: alcohol or sugar pills, perhaps. But then they're simply selling pure alcohol or just plain sugar pills. In other words, Homeopathic preparations are nothing more than a placebo.
Homeopaths claim that during the dilution process, if they shake the mixture just right, the properties of the active ingredient are somehow passed into the water, through some sort of magical "water memory". If that's true, then surely the water would still hold the properties of every other substance it's ever come into contact with, wouldn't it? All the water we currently drink could just as easily be considered Homeopathic Dinosaur Urine or Homeopathic Primordial Soup.
But Why do People Still Buy It?
This is a very good question. One that could just as easily be asked about any number of other fake medicinal preparations. The answer seems to be a little complicated.
It appears to be as a result of a cluster of known bugs in the human brain:
- The placebo effect: since we often can't tell the difference between a sham intervention and real medicine, we sometimes feel better after taking a placebo.
- Mistaken causality: the human brain is an extremely sophisticated pattern-recognition engine. It's far too easy for us to assume a causal link between two unrelated instances - the idea that A came before B, therefore A caused B.
- Anecdotal evidence: hearing the stories of other people who felt better after taking Homeopathic remedies, we'll tend to believe them, and then confirmation bias comes into play - remembering the stories and experiences that confirm our beliefs, and ignoring or explaining away the ones that don't.
What's the Harm?
Another good question: if people feel better after taking a placebo, why not just let them take it to make themselves feel better?
In general I would agree with that. The problem comes in when people turn to Homeopathic remedies first, and don't seek proper medical care that could save their lives. There is a growing list of such such instances over at whatstheharm.net.