The joke will become apparent shortly. For now, all you need to know is that I've just lit another cigarette. The packet that once advertised the finest products of old Virginia no longer comforts. "Smoking kills," it says baldly. Someone has even put a black border around the words in case I miss the point.

I don't. I know about smoking. I am a well-informed addict. I understand what "5mg tar, 0.4mg nicotine, 5mg carbon monoxide" promise for lungs, heart, throat, arteries and other essential bits. I know about the tobacco industry, its callousness, cynicism and bare-faced lies. I've even studied the geopolitics. I am a smoker who approves, fervently, of smoking bans. That's one part of the joke.

Here's another. I've just finished that cigarette. Several more will follow before I achieve the 1280 or so words we can agree to print. (The late Norman MacCaig once said he measured poems according to the number of fags involved. I don't aim so high.) When I'm done, I may even allow myself a drink. The label on an overpriced bottle of Amarone doesn't specify the "units" I need to avoid. A relaxed people, the Italians.

So let's - hold on, I need to find my lighter - talk seriously about public health, personal responsibility, liberty and corporate killing. I promised you a joke, didn't I?

Here's an individual - let's call him "IB" - who is chained to smoking, who can never get his units to add up, and who knows better. He knows lots. Unlike most of the saps killed by the fun-loving organ, he even knows where his liver is. He could tell you that tobacco smoke is so unfunny it actually contains cyanide. Get him started and he will give you the brief lecture on tobacco-alcohol amblyopia.

He is not a victim of that one, not yet. Nor does he suffer from tobacco-related hypochondria. His point would be that education, "campaigns", health scares, self-knowledge and remorse do not quite answer the old Dean Martin joke. The gag was not original. It went, with variations, like this: imagine being a tee-totaller and knowing when you got up that sober was the best you would feel all day. Every day.

"IB" should be over spurious glamour by now. He's lost some weight. The dog walks him frequently. Seasons come and go without a drop passing his (not the dog's) lips. Dog still wonders why Man needs a cigarette at the top of the hill, though. Dog has a point.

I live in a society that forbids me to buy drugs. A nuisance. The world says I mustn't have rare, refined opium, but I can have as many crates of special-offer extra-special lager as I can carry. The world tells me coke would destroy my life, but a packet of finest Smoking Kills, at a fiver-and-you're-joking a pack, is tolerable. Deplorable, taxable, but tolerable.

I eat pretty well. Here, in fact, the joke enters its third phase. I do my five-portions-a-day before noon. Vitamins, fibre, roughage, antioxidised pounds of pure organic goodness: if I pass on before 1280 words tick up, they can weave my innards into a fireside rug. Then, though, I have a smoke, then a drink. If "health indicators" count as opinion polls, my body is ambivalent. But it stays on the right side of the law.

On the right, decent side is a society in which all our grandchildren will explode come 2050. I paraphrase slightly. According to the Foresight report, half the entire population will be obese within 25 years. Snack on that. The cost, they say, will be incalculable. Or rather, because someone has taken a minimally educated guess at a figure, the cost to Britain will be £45 billion. That's a lot of sausage rolls. It makes global warming sound like an excuse for a barbecue.

The avoirdupois apocalypse is not without its compensations, however. According to serious people, if current trends continue we will all be too drunk to notice dodgy eating habits. Thus, amid last week's terrifying tales, came the revelation that "hazardous" drinking is the curse of the middling classes.

That sort of science is even funnier than my cigarette habit. Anyone who drinks two glasses needs to know two things. First, alcohol intoxicates. Honest. Drinking while avoiding intoxication is like arguing while avoiding logic. Secondly: don't be silly. The two-glass, possibly/maybe one-a-day-if-unlucky hazardous drinker is the passive smoker of the alcohol epidemic. The risk exists, but it is barely worth discussing.

Actually worth discussing is the thought that one in five Scots harbours a booze habit. Last week, Scottish Health Action on Alcohol Problems reported that "common drink habits" - not the demonised binge or the recognised addict - are wrecking the nation's health, slowly, surely. Forty-seven percent of men and 36% of women exceed their respective 3-4/2-3 unit allowance in any given week. That's just the honest indulgers.

The reality is far worse. Future archaeologists will wonder why supermarkets allowed so much space to bargain-price dangerous drugs. They will wonder why it took so long to catch on to the fact that many people were pissed, much of the time, and that more shelves were filled with shiny bottles than with fruit or veg.

Fat. Drunk. Drugged into trough-guzzling addictions by never-naive corporations: a mature democracy, then. The best joke, not mine, is that none of this amounts to a "worrying trend", or an exceptional circumstance. It is average, commonplace, and the fat kid's sozzled, farting, colonically compromised future. We really should do something.

Me, I would ban things. Treat the supermarket chains like pushers. Treat them as worse than pushers, if actual human damage is your yardstick. End booze "promotions", hoist the cost of a flame-grilled gut-destroyer towards the prohibitive, enforce a legal obligation on parents to stop poisoning children with whatever is quick'n'tasty. And treat the relationship between political parties and their "food industry" sugar daddies as real corruption.

It would make sense, I think. That alone tells you why it will never happen. Stupendously depressing health outcomes, astonishing social and economic costs, clear correlations at every turn: these things are obvious. A society that cannot take control of obvious, in-your-artery risks to health is stupefied, vulnerable, and easy prey.

Why is it impossible to merely say that a certain meat-related, food-like substance will probably kill more youngsters than E coli? Why do we fail to understand that feeding kids with garbage is much the same, in terms of neglect, as allowing them to be shoved up chimneys? We are corrupted by excess, very possibly, and we are wary, rightly, of granting governments a licence to ban. But why is the parallel between heroin and moreish sugar snacks overlooked? The sales process is identical. I can't smoke in a public place, but I can drink my liver into submission. That's good, or it's bad, but it is certainly perverse.

I've just finished the packet, though. Joke done. The entire point of the last cigarette is to send you in search of the next, and the next. Literally, physically, lethally, that's consumerism. Sophisticate the language and you could call it marketing, or mere peddling.

We legislate, as often as not, to defeat the worst in ourselves. The purveyors of booze and foodstuffs conceal themselves, meanwhile, behind the myths of rare burgundy, fine Havanas and prime beef, as though to justify alcopops, fags and burgers. We do not legislate often enough, though, perhaps because billions of bloated junkies don't quite grasp the trouble they are in.

I make that 1285 words. My habit says (1289) that I'm due another smoke. It knows best.

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