Medicine that doesn't work -- homeopathy

Practice: Homeopathy

In 15 words or less: Like cures like; taking a very small amount of something bad cures related ills.

Initial reaction: Might work sometimes, similar to antivenins or vaccines.

What it really is: Say you have a headache, maybe caused by clogged sinuses. The homeopathic cure is to take something else that causes headaches--almost anything, no matter how unrelated--and dilute it in water by a factor of 10²⁰ to 10²⁰⁰ (or sometimes more). The claim is that the more diluted the solution is, the more effective the cure will be.

Why they still say it works: Water has a "memory" of what chemicals it used to contain. Sometimes the word "quantum" is thrown around, too.

Why it doesn't work: This is going to be a long one.

Homeopathy started with a doctor in the days when they still used leeches to cure colds and were afraid of witchcraft. They used very dangerous medicines in those days, some of which had a reasonably high probability of killing patients. So this guy started diluting his medicines and noticed that more of his patients recovered. Hallelujah! He didn't compare the recovery rate of people taking homeopathic medicines to the rate of people who took no medicine at all (though he might have discovered the placebo effect if he had), and instead came up with the conclusion that "like cures like."

Sadly, water has no memory. Liquids are chaotic things, where the atoms are bouncing around like crazy. That's why they can flow. There is no internal structure that could possibly hold a memory of what was in there before.

And even if that was true, why should taking a poison that gives you a fever relieve a fever caused by a virus? It's like stepping lightly on an infected toe and hoping that that will stop it hurting. Obviously it's not going to work.

Then there are the dilutions. Some people arguing against homeopathy will say that not one molecule of the original "medicinal" ingredient is in the final solution. I disagree with this: there is almost always going to be some contamination from one dilution to the next, from not wiping the rim of the flask carefully or whatever. Anyway, say you use mercury as your ingredient. Do you think there's any chance that not a single atom of mercury ended up in that solution from some outside source? No. Even if you use distilled water, you're going to get a few atoms of whatever was originally in it. So the solutions aren't as diluted as they claim, and this contamination ruins any chance of a memory effect.

Say you did a dilution of 10¹⁰⁰. You are claiming that for every atom in your original solution, you have effectively placed it in 10¹⁰⁰ atoms of solution, and then taken a sample from that. How big is 10¹⁰⁰? It is about 100,000,000,000,000,000,000 times as many atoms as there are in the Universe.

On the other hand, the amount of the medicinal ingredient actually present is incomprehensibly tiny. 18 grams of water has 60,000,000,000,000,000,000,000 atoms in it. If you're saying that there are only a few atoms in there, and that those are making a significant difference in the behaviour of that solution, then... well, if you had $60,000,000,000,000,000,000,000 in the bank, would saving a couple bucks on your new car be a priority for you? You could never even spend that much money, no matter how hard you tried. The antivenin/vaccine comparison is ridiculous.

So, the reasons it doesn't work are many. The medicinal ingredients don't have any reason to work. The dilution factor is too big for it to be plausible. There is more contamination than claimed.

And of course, it's easy to test. Down a bottle of homeopathic pills. It's been done many times, and no one has ever overdosed.

Reources:
There is a good article and many resources at QuackWatch. There is a line there which I find particularly amusing.

In 1994, the journal Pediatrics published an article claiming that homeopathic treatment had been demonstrated to be effective against mild cases of diarrhea among Nicaraguan children.... There was no public health significance because the only remedy needed for mild childhood diarrhea is adequate fluid intake to prevent or correct dehydration.

Dehydration is the one and only condition that homeopathy might have a chance of treating!