It is not known why some people develop acne, although heredity may play a part. Equally, it is not clear why the incidence of late-onset acne is increasing. One of the main causes is thought to be stress, which increases the production of male hormones (androgens). Greater use of oral contraceptives, which contain progesterone, may also be a factor.

Hormone changes before menstruation and during pregnancy can cause a flare-up of acne, as can hot temperatures and high humidity. Diet plays no part. 'It is a myth that chocolate and fried foods make acne worse,' says Alison Dudley, chief executive of the Acne Support Group. 'However, a good balanced diet will improve your general wellbeing and this will be reflected in the condition of your skin.'

Another myth is that acne is caused by poor hygiene. If anything, sufferers tend to wash more frequently than those with clear skin in order to clear away excess oil. This may be a mistake as over-washing can dry out the skin and make the condition worse. The Acne Support Group advises using a pH balanced cleansing bar or facial wash twice a day. Washes aimed at oily skins can be over-harsh, but moisturisers for oily skins are beneficial as they provide hydration without extra oil. Keep make-up to a minimum. When you do use it, choose oil-free cosmetics and make sure you cleanse thoroughly before bed.

There are numerous over-the-counter treatments for acne. Few are medically proven and what works for one person may not work for another. Clinical trials suggest that tea tree oil may be effective, though it can dry the skin. The trace element silicic acid has also been shown in trials to reduce spots.

A wide range of stronger treatments is available on prescription. Once again, you may have to try several before you find a product that works for you. As a general rule, if you do not see a significant improvement after two months, ask to try something else.

The first-line treatment for mild to moderate acne is usually a topical cream or gel containing benzoyl perodixide, which reduces oil production and/or antibiotics, which lower bacteria levels. For blackheads, a product containing Retin A, a vitamin A derivative that works on the development of skin cells, may be more effective although it can make fair or sensitive skin red and sore.

If topical treatments do not work, you may be prescribed an oral antibiotic. You will normally be given a six-month course, sometimes starting with a high dose, which is later reduced. The most popular antibiotic, tetracycline, can interfere with the effectiveness of the contraceptive pill and may alter vaginal microflora, causing thrush.

There are a number of other oral treatments for acne, including Dianette, a pill containing oestrogen and cyproterone acetate that combats the effects of testosterone. The most powerful drug available is Roaccutane (isotretinoin), which has revolutionised the treatment of the condition. It is highly effective but has some unpleasant side-effects, including joint or muscle pain and nose bleeds, and can only be prescribed through a consultant dermatologist. Women must not become pregnant while taking Roaccutane, or within a month of stopping it.

The latest treatment for acne is a lightbox that uses fluorescent lamps to shine blue and red wavebands of light onto the skin. Three-quarters of the patients who took part in a clinical trial at the Hammersmith Hospital in London using the lightbox for 15 minutes a day over an eight to 12 week period reported a significant improvement in their spots.