Acne is caused by blockage of the pores or sebaceous glands. These glands start to produce sebum - a greasy substance - at around puberty, and the pores get blocked by this. Sometimes a small black spot of pigment called melanin appears on the top - the blackhead. The pores can also get infected with bacteria, which may cause inflammation.

Who gets it?

About 90 per cent of teenagers get spots at some point. You are more likely to suffer from acne if your skin is naturally greasy, but you will probably grow out of acne naturally by the time you leave your teens. You may find it gets worse if you have a lot of stress, such as around exam time. If you are a girl, you may be more prone to acne just before your period.

There are lots of theories about what makes acne worse, but most of these are just myths. Your diet - eating too much chocolate or greasy food, for instance - is unlikely to make acne worse. It is not caused by being dirty and never washing.

What are the symptoms?

You may start getting spots at puberty. Acne is particularly troublesome because it comes at a time when your appearance is especially important to you. Most of the spots will disappear without trace after a few days, but some nasty spots can leave scars.

If you are black, you may occasionally be prone to developing thick scars called keloid scars.

If you have spots on your forehead, you may be tempted to grow a fringe to cover them.

Unfortunately, this will probably make them worse. If you wear make-up or spot concealer, you may find the spots get worse if you do not wash the make-up or concealer off regularly at the end of the day.

When should I go to my GP?

There are lots of products available from the chemist to help with acne. Some contain benzoyl peroxide, which can be very effective. This acts by peeling away a very thin layer of skin, so it may make your face red when you first use it. This should soon settle down, so it is worth carrying on trying. If your acne does not respond to these cleansers once you have used them regularly for several weeks, you may want to make an appointment to talk to your GP. You should also see your doctor if you find the spots are painful or start leaving scars.

What is the short-term treatment?

Most treatments for acne take several weeks to have a good effect. Your GP may recommend you start, or carry on, using a lotion on your face. He or she may also add in antibiotic tablets for you to take. Certain kinds of contraceptive Pills are effective for acne in girls, and your doctor may recommend one of these.

Will I need long-term treatment?

Acne does usually go away by itself eventually. But you may need to take treatment, either continuously or for a few months at a time, for some years.

How can I stop it getting worse?

You should use cleansers regularly, and take any treatment your doctor gives you regularly. Try to avoid getting too tired, stressed or run-down.

Will I have to go to hospital?

Some people get acne so badly, the usual treatments do not work. If your acne remains bad with treatment from your GP, or if you have scarring due to it, your GP may refer you to the hospital to see a specialist