When I was growing up, the hard sell in adverts was sex. Ladies lying in rippling fields, languidly toying with chocolate bars, mysterious men leaping through windows while clutching a box of Milk Tray, that sort of thing. It was pretty steamy, if pretty harmless, but it worked. Sex sold. There was a bit of sort-of science about: Fairy liquid gave your mother soft hands, nine out of 10 cats preferred a particular brand. But it was rather soft stuff.

These days, however, sex is old hat, and the new hard-sell is science. And properly hard science at that, or so one might innocently presume. Ever since Jennifer Aniston told us that the "science bit" was coming in a commercial for L'Oreal Elvive, there has been an increasingly dazzling proliferation of science in our adverts. "Pure extracts", "spring water concentrate", "unique amino complex" and "a new generation of ingredients" are the kind of thing we now expect to find in our shampoo/face cream/loaf of bread: curiously, "clinical tests" always seem to reveal that 93% of women find said shampoo/face cream/loaf of bread of "proven effectiveness". And if an advert says something is 93% effective, that must be right, mustn't it? After all, you can't just go on telly and make up statistics, can you?

Well, no, you can't. Yesterday the French cosmetics giant L'Oreal was forced by the Advertising Standards Authority to withdraw a major ad campaign after making claims for two products, Anti-Wrinkle De-Crease face cream and Perfect Slim anti-cellulite cream, that it couldn't back up scientifically. The TV ads, which starred Claudia Schiffer, claimed that 76% of women had "visibly reduced expression lines" after using Anti-Wrinkle De-Crease, and that 71% of women found that Perfect Slim "visibly reduced the appearance of cellulite". The ASA found there wasn't enough evidence to support either claim, and the ads will now have to be amended. In May, similarly, advertisements by Estée Lauder were also found to be misleading.

So do these companies simply lie? L'Oreal, which made £1.4bn profit last year, insists not. In a statement yesterday the company said it disagreed with the verdict and that any claims it makes for its products are "substantiated by scientific evidence and customer research". Who to believe? What soon becomes clear is that even when a manufacturer is not, strictly speaking, telling an untruth in its advertising, the "science bits" its telling you are highly unlikely to be the full story.

Take Pantene Pro-V, which has recently been telling us, via shiny spreads in various magazines and TV ads, that its Anti-Breakage Shampoo, will lead to "up to 95% less breakage in just 10 days". Small print at the bottom of the page tells us that testers looked at "brushing damage, shampoo and conditioner versus non-conditioning shampoo". This is presented as credible science, but credible science involves doing things in a certain way. How many people, for example, took part in the trial? (Perfect Slim, the ASA found, was tested on just 48 women, meaning that the number of women who noticed a difference was in fact only 34.) Did the participants know what kind of shampoo they were using or were they "blinded" to it, as they would have been in a serious scientific trial, and as did not happen in the case of Perfect Slim? Did the company run a proper comparison of shampoo and conditioner of their brand against both types of product from another brand?

I decide to find out. After several phone calls over several days, I am put through to one of Pantene's senior scientists. How were the tests done, I ask. All methods related to this are commercially sensitive, Dr Steve Sheil says, but he does tell me that he and his team tested 10 samples of hair, three times, with reproducible results. The results were "significant". But how significant can testing 10 hair samples three times really be? Does he really think that this is credible enough science to serve as the foundation for a major, science-based advertising campaign? "All the claims have to be cleared by the Broadcast Advertising Clearing Centre," he says, and here he is quite right. The BACC, which is funded by the advertisers themselves, approves adverts for television by vetting them before they make it on to our screens (it approved the contentious L'Oreal ads). It insists that claims made during television advertising must be substantiated, and it will refer to a body of scientific and medical consultants in contentious cases.

Even so, it's arguable that small studies of this kind, carried out in what amounts to secrecy, aren't much of substantiation for anything. They provide a cloak of scientific credibility, but they don't undergo the analysis that occurs when science appears in the harsher world of scientific publications. How is an ordinary consumer to find out what the research actually involved.

Claire Forbes is director of communications at the Advertising Standards Authority. She says that there are firm guidelines as to what can and cannot be claimed in advertising, but the ASA, with a staff of 100, has no legal standing to regulate advertising. Staff look at all the major newspapers daily, but with an estimated 30m adverts printed every year in the UK, it is impossible for them to look at them all. Instead the authority relies on public complaints, 14,000 of which are made annually. Forbes cites the recent case of a slimming pill whose advertising was withdrawn after making claims that were found to be based on a study on just 44 people. The ASA, after a complaint, decided that this was too small a study to be valid.

"Talking generally, we may accept a small sample size as reasonable proof, but this would really depend on the statistical significance of whatever tests were done," Forbes says. "Conditional claims lead to a host of different claims, especially when 'modal verbs' are used. We might ask them to change 'can' to 'could' if they didn't have 100% proof of the 'can'. But we would also expect them to hold proof relating to the 'could'."

But weasly verb tenses aren't the only problem. It's also the way the research is conducted. Take an advert for another L'Oreal product, Revitalift. In "clinical tests", according to the ad for the cream, of 40 women "93% say their skin felt softer, and 79% say their skin was firmer with each application". In 2004, L'Oreal spent €500m (£340m) on research and development, so you'd expect some really vigorous analysis - perhaps the use of a dummy cream or ordinary moisturiser to act as a contro. I ask Benedicte de Villeneuve, scientific advisor for L'Oreal Paris about this. "Carrying out a placebo-controlled test does not make much sense in our industry as a cosmetic product is a balanced and precise mixture of cosmetic ingredients and its effectiveness relies on this specific combination of ingredients," he emails in response. But why can't not use some bog-standard emollient as a control, so that the consumer at least knows whether this product, or nappy cream, is better?

Discovering the "truth" about hair breakage or skin firmness is not, granted, the stuff of life or death. ln any case, scientific truth is more often about revealing degrees of certainty than it is about finding rare absolutes. But rubbish science in face cream adverts may end up undermining the proper science we read about in the stories next to the adverts. And it is not only beauty products that use sort-of science to sell.

In the UK, several medicines that have come "off prescription" and are now available for over-the-counter purchase use standard medical jargon in their adverts. Do the medicine companies do any better than the cosmetics companies?

Zocor Heart-Pro provides one very good example. This medication, a dose of 10mg of simvastatin, is a type of drug which lowers cholesterol, and has been available to people with heart problems or who have high risks for heart disease for many years. It was previously only available on prescription, but in July 2004, it was reclassified, enabling it to be sold from pharmacies at a cost of £12.99 for a 28-day supply. "Heart disease is the number one killer in the UK," says a spokesman for McNeil, the company which manufactures Zocor, "and any proactive steps that we can take should be applauded." The packaging and promotion say the drug "effectively reduces the risk of a heart attack" by "preventing build up of harmful plaques in your coronary arteries" and "reducing your risk of coronary heart disease".

The problem, however, is that taking simvastatin in the dose at which the drug is available over the counter (10mg) has never been directly proven to be beneficial for people in the moderate risk category, at whom the drug is targeted. Dr Ike Iheanacho, editor of the Drug and Therapeutics Bulletin, a journal published by Which? (formerly known as the Consumers' Association), says: "The key issue is that of evidence. If you don't have evidence to justify claims of benefit, then the whole argument begins to fall apart. And the evidence that this [Zocor Heart-Pro] will significantly reduce the risk of a heart attack in the target group is at best flimsy."

A recent edition of the journal highlighted key concerns about over-the-counter "-statin" drugs. "It's not an attack on statins per se, which are crucial treatment for many patients," Iheanacho says. "But there is a lack of evidence that Zocor Heart-Pro will do any good - its case relies on circumstantial evidence that has been extrapolated. This is being presented as a scientific certainty when it is anything but." He also takes issue with the initial adverts (since changed) for Zocor Heart-Pro which stressed that the risk of a heart attack was up to "one in seven", without pointing out that that this risk was over a 10-year period. "Even health professionals may have been confused by this omission. So how was the general public not to know that this didn't mean, for example, that there was a one in seven chance of dying of a heart attack the next day? In our view, giving examples of risk selectively in this way is impossible to defend."

The Medicines and Healthcare Products Regulatory Agency (MHRA) is responsible for ensuring that medicines advertising is not misleading. In a statement, the agency said that it had considered a complaint about the way the risk of heart attack was expressed by the Zocor Heart-Pro advertisements, but did not uphold it. "While acknowledging that how risk is expressed and how it is understood is a difficult issue, with the need to strike a balance between unduly reassuring and alarmist messages, the agency took the view that the advertisements encouraged people to seek advice from their pharmacist who is best placed to give a fuller explanation of the level of risk and alternatives for reducing it."

Perhaps people will, when it comes to medicines. They almost certainly won't seek advice over yoghurt drinks, for example, which are suggested as an aid to "digestive health". There is scant evidence that drinking these kinds of drinks will put one's digestion back on track. Yet the claims made by the manufacturers are so vague, how are we to object to them.

Is this really good enough? Beauty cream consumers may tread a precarious balance between the sceptical and the credulous, but in medicine and food advertising too, buyers should beware. Marketing and science have got together and bred a weird hybrid form of sales-experiments that have taken over our advertising culture. The next time they get to "the science bit", don't forget to add a pinch of salt. As L'Oreal's customers have discovered, much of what we are told might just as easily be science fiction.