'Vitamin injections are meant for treatment of specific vitamin deficiencies only. This is rare as a lot of processed foods already have various vitamins added,' says Michael Cheng, a doctor at the Premier Medical Centre in Central.

So the obvious question is, why are more and more people injecting their vitamins?

Mandy D'Abo, owner of the Cat Street Gallery on Hollywood Road, had her first vitamin B12 injection to counter overwhelming fatigue.

'I had some blood tests and my doctor saw I was super-low in vitamin B,' she says. 'You know the feeling - when you go to bed early but you still wake up and can't shake the tiredness.

'The doctor put me on a strong dose of oral vitamin B and also recommended a course of three vitamin B injections to get my levels back up to normal. I definitely felt a lot better. But I believe you've got to do both to feel the difference - they go hand in hand. I don't think they work in isolation.'

Lauren Bramley, a doctor in private practice in Central, says that although vitamin injections are nothing new to medicine, there is a much better understanding of the role they can play.

'Vitamin B12 injections are a way to quickly improve B12 levels, which are very often found to be at the low end of the range due to stress, poor B12 intake or poor absorption due to digestive problems,' she says. 'B12 injections can contribute to treatments for stress and anxiety, sleep disorders, depression, back pain and cognitive problems'.

A study last year at Oxford University found a direct correlation between brain volume and B12 levels in 70-year-old men: those with larger brain sizes had higher levels of the vitamin. The study found that subjects who had higher B12 levels were six times less likely to experience brain shrinkage than those who had lower levels.

Bramley says vitamin injections can also be used to treat poor immune function, fibromyalgia, chronic fatigue syndrome, adrenal fatigue, anaemia, low back pain and obesity, and they can slow the ageing process by improving collagen, treating wrinkles and pigmentation problems and boosting vitality.

Author and New York 'It' girl Plum Sykes attributes her flawless skin to vitamin C injections, which also include small amounts of vitamins A, B and E, with zinc and copper. Sykes is not alone - a long list of celebrities including Jade Jagger and Fergie are also fans of the addictive C jab.

'Vitamin C is a very potent antioxidant in the free-radical theory of ageing, by slowing down telomere [shortening of our genes] and extending longevity of our cells,' says Bramley. 'Vitamin C, along with a blend of other antioxidants, as well as hyaluronic acids, can be injected directly into the skin as mesotherapy, as a treatment for collagen improvement, stubborn pigmentation, cellulite and stretch marks.'

And the benefits of vitamin C injections go beyond mere beauty enhancement - apparently they can also help with a range of health problems, including allergies. Office administrator Liz Storey was administered with a vitamin C drip at a Hong Kong medical practice to help her recover from a chronic case of hives.

'I had several non-conclusive allergy tests, so my doctor suggested trying a vitamin C drip,' says the 39-year-old. 'It's a powerful antioxidant and I obviously had a high dose of histamine in my body which was caused by a reaction to something.

'In the end we concluded it was an allergy to nickel - studs in my jeans, buckles in my shoes, the underwire in my bra - and my reaction every time I was exposed to it was to come out in hives.

'The only way to break the cycle was to have a high dose of vitamin C, which you can't take orally because it will give you diarrhoea. It's controversial, but there are no known side effects. The vitamin C mops up the histamine, which was going crazy. It was remarkable: the hives stopped after only three sessions and just disappeared. I haven't had any for almost three months.'

It all sounds just too good to be true - so what's the downside? Like anything, it's open to abuse; a lot of the controversy surrounding vitamin injections centres on the dangers of injecting fat-soluble vitamins such as vitamins A, E and D (said to offer a range of benefits from slowing ageing to helping prevent cancer) that are surprisingly easy to buy on the internet.

Experts agree that injections should be administered only with proper medical supervision. 'The risk is that fat-soluble vitamins [A, D, E and K] may accumulate in the body - water soluble vitamins [C and B] are usually removed via urine. Excessive amounts of vitamins are toxic and can cause side effects,' says Cheng. 'However, the safety margin is quite high.'

Evidence shows that taking fat-soluble vitamins in tablet form isn't usually a problem, because the digestive tract acts as a filter, preventing the risk of overdose.

Claire MacEvilly, a nutrition scientist at the British Nutrition Foundation, is more sceptical. Quoted extensively in the British press, she warns: 'Vitamin injections are an extreme and dangerous fad which has come from the US. Overdosing on non-soluble vitamins by injecting them is easy to do and it is a real concern. It can cause cramps, nosebleeds, nausea, blurred vision, dry skin, liver disease, weight loss, kidney stones and permanent kidney damage, irritability and jaundice.'