He is worried about keeping it up. She is worried about keeping it down. He thinks he's a flop in bed. She thinks she's too fat. But now their worries are over. This is the age of... His 'n' her pills

There is nothing he wouldn't give to look down and see a bulge below his belt. But if she looks into the mirror and sees a bulge below, above or underneath her belt, she's in instant hysterics and off to the gym to do 1001 sit-ups in a tent dress. Limp penises and spare tyres - these are the twin nightmares of our age. No man can be too hard, no woman too thin.

There is a terrible symmetry to the way these two cultural injunctions blight marriages. For every husband who suffers from impotence, there is a wife who thinks it is her fault because she suffers from size 14. But not for much longer. Welcome to the age of his and her drugs.

You've already heard half the story. You probably think you've heard too much. There must have been a moment some time last summer when you put down your paper, shook your head, and asked: "What will they think of next?" Well, it didn't take them long. The official name for the new miracle pill is Xenical, but people are already calling it the female Viagra.

Xenical is the first weight-loss drug that doesn't act on the brain as an appetite depressant. Instead, it works in the intestines, where it keeps 30 per cent of the fat you eat from getting absorbed. This means that you can enjoy every bite you eat without having to wear it. In the trials, patients had an average weight loss of 10 per cent within one year. This is wonderful news, the doctors of the world insist, because obesity is becoming such a big health problem. Right now, 13 per cent of men and 16 per cent of women in Britain are classed as obese. By 2005, the figures are supposed to go up to 18 per cent for men and - I can hardly bear to type this! Where do these figures come from? - 24 per cent for women.

So Xenical is not just a miracle, but an eleventh hour miracle. It is not, however, to be used in a careless, millennial fashion. You're supposed to take it in conjunction with the usual sort of sensible treadmill and low fat yoghurt diet. And it's for the truly obese - not women who know they will be vastly more attractive if they can shed seven to 10 pounds. But even Hoffman-La Roche is admitting that it is not going to be able to stop them from getting it if they really want it.

They admit, too, that according to most surveys, all women think they're seven to 10 pounds fatter than they should be. So you can imagine what kind of a stampede we're going to see. And if the risk of dying from a heart attack has not stopped men from rushing to buy Viagra, then it's hard to see why women would be deterred from purchasing Xenical because some American experts think the drug might impede the absorption of vitamins, or might lead to an increased susceptibility to breast cancer. There are no free lunches, and it isn't every day that the dowdy and the middle- aged get a chance for a second adolescence. So we might as well get ready for the bumpy ride that is ahead.

But let me tell you: I am dreading it. Family loyalty prevents me from going into details, so let's just say that I know a 75-year-old man who's been taking Viagra for a year. It's turned him into an emotional adolescent. He can think of one thing and one thing only. He's forgotten everything else - Christmas, birthdays, grandchildren, manners, bills, taxes, propriety. Having a conversation with him is like going to a blue movie and any family member who complains about this risks getting struck from his will. The only thing that is hemming him in right now, and saving him from all-out depravity, is the shortage of suitable playmates. Just imagine what we're in for, if in a year's time the streets of his small town are packed with bored, but suddenly slim, housewives.

How will we stop them from losing their dignity, and forgetting their other duties - for example, the importance of setting a good example for their grandchildren? Senior Viagrans tend to be merciless on this point. They say: "You had your sexual revolution, now you can let us have ours." They laugh dismissively when their children explain that it's no fun to watch. But who knows? Maybe this is just the first stage of the second sexual revolution. Maybe in a year's time, we will be so busy packing cream cakes for our own naughty weekends that we won't even notice.

But before we go, perhaps we should ask ourselves why the inventors of miracle cures have such an easy time seducing us. We are none of us new to this game. After all, we live in the century of the quick fix. Valium and Thalidomide, speed, the Pill and Prozac - they all arrived on the back of million and billion pound publicity campaigns, to save us from ailments that were once just "part of life".

Each time there was a new one, we were assured that this time, there would be no side-effects. And, of course, there always were. We helped the manufacturers understand more about the shortcomings of their drugs by offering ourselves up as guinea pigs. And even though the experience turned us all into cynics, it has not stopped us swallowing.

We know that these new drugs are going to do us untold harm - not just in the ways we've already been warned about, but in other ways none of us can yet imagine. It's the never intended social consequences that we really need to be thinking about. Now that we have 25 years of sexual revolution behind us, we all know only too well that quick fixes do not cure people of their cultural prejudices and only serve to ossify them.

This has always been easier to see in other countries. Everybody was delighted when technology gave us a way of finding out which sex a foetus was. And then they were horrified when, in certain parts of the world, people used this technology to identify and abort those that were female.

We can see the same process at work here. Convinced that we have to be fit, that is, thin, we run for miles - and incur osteoporosis and heart failure as our reward. Even knowing the potential dangers doesn't stop us. So fixated are we on the "perfect" physical model, we seem literally willing to die for it.

Like all chemical cures, Viagra and Xenical will save us from one set of neuroses only to replace them with others. The chemists at Hoffman- La Roche have already anticipated these problems, and the pills that will chase them away.

Quick fix chemical cures are based on an impoverished view of what life is about. One day it should be possible for men to be erect and unstoppable, and women to be thin and irresistible, without a single man, woman, or child having to pay a social cost. We will turn Viagra and Xenical into smart weapons by using them not just in conjunction with exercise and low-fat diets, but also in alternation with pills that: make you enjoy housekeeping; keep your eyes on the road during the school run; help you not look down your secretary's blouse; help you forget what you got up to last time so that you don't have to feel guilty about it; remember who you're married to; and even pills to remember why you married them.

But imagine how hard it will be to remember which pill is which. Imagine the havoc you could cause by taking them in the wrong order! Soon we'll be so confused about who we are and how we got here and what we did when we were there, that we'll look back on the sexual scandals of today, and wax nostalgic about their moral certainties.

We'll think of Clinton in the same way we think today about women who got married just before the pill came on the market, and so missed out on the Sixties. "Poor thing," we'll say. "He was born ahead of his time." There will be a wider ripple effect: the more women there are out there who are acting badly, the more we will be forced to forgive men, and ourselves, when we like typical men. Greater humility in the face of human error - that's the only good thing I can see coming out of this revolution. The rest of it is going to be terrible. I can't wait.