Why the hair of the dog is the best hangover cure

 From the book's introduction:

I've had perfect bar moments. They're what led to this book. Here's one: I was supposed to meet a friend for an after-work drink on a swamp-sticky Washington, D.C., summer day, and I was late. I rushed across town to get to the bar and showed up a mess, the armpits of my shirt wet, hair stuck to my forehead.
The bar, though, was cool and dry—not just air-conditioner cool, but cool like they were piping in an evening from late autumn. The sun hadn't set, but inside, the dark wood paneling managed to evoke 10 p.m. In a good bar, it is always 10 p.m.
I asked for a beer; I don't remember which one. The bartender nodded, and time slowed down. He put a square napkin in front of me, grabbed a pint glass, and went to the taps. He pulled a lever, and beer streamed out of a spigot. The bartender put the glass of beer in front of me, its sides frosting with condensation. I grabbed it, felt the cold in my hand, felt its weight as I lifted it. I took a sip.
Time stopped. The world pivoted. It seems like a small transaction—a guy walks into a bar, right?—but it is the fulcrum on which this book rests, and it is the single most important event in human history. It happens thousands of times a day around the world, maybe millions, yet it is the culmination of human achievement, of human science and apprehension of the natural and technical world. Some archaeologists and anthropologists have argued that the production of beer induced human beings to settle down and develop permanent agriculture—to literally put down roots and cultivate grains instead of roam nomadically. The manufacture of alcohol was, arguably, the social and economic revolution that allowed Homo sapiens to become civilized human beings. It's the apotheosis of human life on earth. It's a miracle.

A new book on the science of alcohol has the reviewers concentrating on the suggestion that the best cure for a hangover is another drink. The hair of the dog theory is based on the possibility that because every alcoholic drink contains, as well as ethanol, small traces of the poisonous methanol, doing something about that residual methanol is what helps recovery. And how best to do that?
In high doses methanol can make people go blind or even die because the body converts it to formaldehyde, a poison used as a preservative in some laboratories.Doctors treat methanol poisoning by giving patients ethanol to prevent its change into formaldehyde. 'If methanol poisoning is at least in part responsible for a hangover, having a drink the next morning may alleviate symptoms,' said Mr Rogers although he was at pains to point out that the theory was 'hypothetical' at best.