It is something of a mystery how Hilary Swank has managed to find the time to forge a Hollywood career and win two Oscars, not to mention fitting in the more everyday business of popping out to the shops or taking her dog for a walk, given the amount of time she must devote to her extensive pill-taking regime.

In a recent cover interview with American style bible, W magazine, the actor breathily regaled the journalist (who was possibly wondering how you steer a Hollywood heavyweight away from the subject of capsules and onto more meaty tell-all celebrity interview fodder) with the finer details of all the brilliant nutritional supplements she relies upon.

"This is my Aloe C," she began (Aloe C, as the name suggests, is a combination of Aloe Vera and vitamin C). "Here's my flax. This one's for my immune system. And this one is my BrainWave." BrainWave is designed to enhance mental function through a balance of "smart nutrients". It hasn't been evaluated by the Food and Drug Administration in the US, but Swank is already convinced. "It's great, like if I have a lot of lines to memorise." Or a lot of pills to memorise, since she continues to rattle off a list of the 45 supplements she takes every day. "I just took my most important ones," she concluded, "which are my Oz Garcia Longevity Pak."

An Oz Garcia Longevity Pak, if you're wondering, contains six capsules packed with "powerful nutraceuticals". (Oz Garcia, it turns out, is a "life extension specialist" dubbed on his website, "Nutritionist to the Stars".) It is also, after the W interview, out of stock despite the regime costing pill-poppers $90 a month.

But while Swank is doubtless at the more extreme end of the supplement-taking spectrum, there can be few people who can honestly say there aren't at least a couple of dusty pots of multi-vitamins or fish oil cluttering up their cupboards. But are we - Swank included - really doing ourselves any good by taking any of them?

In some cases, taking a nutritional supplement can serve a specific purpose. Sue Baic, a dietician and spokesperson for the British Dietetic Association, says that people with enhanced needs for certain nutrients, or whose diets are deficient in some way, will benefit from taking tablets. "Pregnant women, or women who are planning to become pregnant, could take folic acid," she says. "Calcium is good for people at risk of osteoporosis, vitamin D for breastfeeding women or children under two, and B12 for strict vegans."

For most of us though, Baic says, "it's not hugely difficult to get all the vitamins you need from your diet, and the people who take the most supplements tend to be the people who need them least. People who spend a lot of time thinking about vitamins generally have the best diets."

Dosing yourself up on a range of miracle-promising supplements is, at best, pointless and expensive. "The excess nutrients will just be excreted," says Baic. Which is something Swank might do well to ponder when she notes that her beloved Longevity Pak contains 1,667% of the recommended daily value of thiamin, 1,470% of riboflavin and 3,333% of B12. This last is especially pointless - though understandably tempting.

B12 is readily available in food, but in large doses has been observed to give an energy boost to those receiving it. But, alas, due to the way it is absorbed by the body you can't simply gorge yourself on the stuff, sit back and await a rush of get-up-and-go. You'll absorb as much as you need and get rid of the rest. Which is perhaps why the W interviewer notes Hilary dashing off during their chat to get a vitamin shot from a visiting nurse, since an injection packs a bigger B12 punch.

Dr Steve Hajioff, a GP who also works at the Alive and Well clinic in Covent Garden, London, administers B12 shots to his patients and argues that they are a useful standby for people with busy or tiring lives. "They have been used in the entertainment industry for years," he says, "and I've heard that Margaret Thatcher used to have one every day."

But Hajioff points out that vitamin jabs aren't really about making up for a deficiency. "By having a tablet you'd only get the minimum dose because that's all that would be absorbed. But an injection gets the higher dose into the bloodstream. They've been observed to give a boost to energy and focus that lasts for a couple of weeks, and I'd say it works for about nine out of 10 people."

But even Dr Hajioff wouldn't recommend a regime of regular B12 injections. "It's a quick fix, a crutch," he says, "and you shouldn't use a crutch more than a couple of times. I won't do it for a patient more frequently than every two weeks and I won't do it more than two or three times without looking at other factors, like lifestyle and other underlying health problems."

Baic cannot see the need even for "quick fix" supplements. "It does worry me," she says. "Injections are an invasive process. If you haven't got a genuine deficiency, why have a jab? It's not hard to get the amount of B12 you need if you eat meat, eggs or dairy foods." There's also no hard evidence that B12 injections even work and, at between £60 and £100 a shot, it's an expensive route to a placebo effect.

A check on Swank's diary - "In the past four-and-a-half weeks, I've been to London, Texas, Chicago, China, New York, LA and New York again" - might make you wonder if some people's lifestyles are simply too packed to do anything but rely on nutrients in pill form, but Baic has little time for that argument. "Presumably she still has to eat," she points out. "And she'd be better off thinking more about what she eats than all the tablets she's taking."

There is evidence to suggest that nutrients in their natural state are more effective. "We know that fruit and vegetables can help protect against cancer," says Baic. "But remove the antioxidant ingredients from the food and just take them as supplements and they appear to have no benefits. The supplements we can buy are only the ones that have been identified as necessary - there are probably others [in food] we don't know about."

Besides, too much of a good thing can be bad for you. "Beta-carotene supplements have been linked to an increase in lung cancer," says Baic. "Vitamin A can be really dangerous in high doses, B6 can cause nerve damage, folic acid can mask a B12 deficiency and high doses of some nutrients can inhibit the absorption of others."

None of these problems should arise with a normal, healthy diet - even one supplemented with a standard multi-vitamin. But swallow 45 a day and you might be drifting towards danger, and you're certainly wasting a lot of time and money. And that's before we even mention the jeopardy that Swank has already identified after admitting she's just shoved a load in her mouth, "which I actually shouldn't do, because I choked on my vitamins once before".