More than 22 million prescriptions were written for sufferers of depression in England a year ago, compared to nine million in 1991. Dr Lesley Hickin describes the three main groups

Our subjective moods of happiness and sadness relating to daily events are usually fairly well balanced and appropriate to what is going on. Sometimes however, we can feel low for no apparent reason. If this is accompanied by other features it may be part of an episode of depression. (See Depression and manic depression).

Depression is one of the most common symptoms affecting your mood. Many people feel down for various periods of time but if it interferes with your ability to carry on a normal daily life, or lasts longer than two weeks, you should talk to someone about how you are feeling. This may be a close friend or relative, or your GP, nurse or health visitor. Depending on your previous experience of your health care professionals you may know who will be a sympathetic ear.

People suffering from depression need help. If they could 'snap out of it' or 'pull themselves together', believe me, they would. Being severely depressed is a life-threatening illness; people who don't get the appropriate help are at risk of committing suicide.

Therapy or antidepressant?
In mild and moderate cases psychological treatments (the talking therapies) can be very effective. In severe depression or when there are a lot of physical symptoms, then antidepressant drugs are invaluable. They help relieve many of the symptoms such as despair, lethargy, poor appetite, insomnia and thoughts of suicide. They are effective in about two out of three people who take them.

It is currently believed that people who are depressed have altered levels of some of the chemical messengers in the brain. These are also known as neurotransmitters.

The two principal chemical compounds involved are noradrenaline and serotonin. When present in normal amounts these are thought to increase brain activity and improve mood, but they are found in reduced amounts when people are suffering from depression. This is thought to lead to the transmission of faulty messages and to be responsible for some of the symptoms of depression. Drugs that increase the levels of noradrenaline and serotonin are used to successfully treat the illness.

The main types of antidepressants
Antidepressant drugs are taken by mouth in tablet or liquid form and usually take one to three weeks to start having an effect. Side effects may happen more rapidly, but gradually lessen, so it is worth persevering with your medication. Sometimes you may be started on a low dose to get used to the side effects, but the sooner you are on an effective dose of the drug the more quickly you will start to feel better.

Most of the drugs used to treat depression belong to three main groups:

1. SSRIs (Selective serotonin reuptake inhibitors)
Group includes: Fluoxetine, paroxetine fluvoxamine, sertraline and citalopram. You may know them by the 'trade name' given by the drug company that markets the drug, for example, Prozac, Seroxat, Faverin, Lustral and Cipramil.
Used to treat: SSRIs are now the most commonly prescribed type of antidepressant drugs, and are also used to treat phobias and panic attacks. They are also extremely useful in higher doses for eating disorders such as bulimia and anorexia nervosa.
How they work: Nerve cells in the brain constantly produce, release and reabsorb the chemical serotonin. This acts on brain cells to stimulate increased brain activity and improve mood. SSRIs prevent the reabsorption of serotonin, resulting in increased levels that are free to act on the brain cells and increase stimulation.
Side effects: SSRIs cause fewer unwanted effects (side effects) than other classes of antidepressant, and are less toxic in overdose. Unwanted effects are most apparent at the beginning of a course of treatment and include nausea, diarrhoea, reduced sex drive and headache. They can also cause restlessness and anxiety.

2. TCAs (Tricyclic antidepressants)
Group includes: Amitriptyline, clomipramine, imipramine and dothiepin. The term 'Tricyclics' refers to the shape of the molecule of the drug.
Used to treat: Depression, pain from damaged nerves, tension headaches and bed-wetting in children.
How they work: They interfere with the reabsorption of both noradrenaline and serotonin, and as a result levels of these mood-lifting chemicals increase.
Side effects: They have more side effects than SSRIs, such as dry mouth, blurred vision, constipation and difficulty passing urine. Side effects are usually worst at the beginning of a course of treatment, so it is worth persevering with the drugs. Some doctors prescribe a low dose initially and increase it gradually.

3. MAOIs (Monoamine oxidase inhibitors)
Group includes: Phenelzine and tranylcypromine.
Used to treat: These are used less commonly nowadays and generally when other drugs have failed. They are being superceded by more modern antidepressants.
How they work: They work by blocking the breakdown of serotonin and noradrenaline in brain cells.
Side effects: These include light-headedness, drowsiness or insomnia, headache, dry mouth and digestive problems. They interact with some foods and other drugs which can lead to dangerous changes in blood pressure.

Other options

• Other newer drugs are available that work on both neurotransmitters, such as venlafaxine, maprotiline and trazodone.
• Some people with recurring episodes of depression are treated with mood-stabilising drugs such as lithium, carbamazepine as well as an antidepressant.
• Many people are keen to try herbal remedies and St John's Wort has been marketed as an antidepressant for use in mild depression. You should be aware that there is no standardised concentration of the active ingredient in the various preparations, so you can get different doses if you switch brands. You should always tell your doctor if you plan to take St John's Wort because it can interfere with the breakdown of other drugs such as drugs for epilepsy, the contraceptive pill and some heart medication.

How long should treatment last?
It usually takes two to three weeks for an antidepressant to start working. Your doctor will probably suggest you take it for at least six to eight weeks before you can be sure whether or not it is helping. If that particular one doesn't help you may try a higher dose or switch to another drug.

Most people are used to taking a course of treatment for a certain time, then stopping when they feel better. But depression is different - it is better to carry on with your medication for several months. If you don't do this you have a one in three risk of becoming ill again within six months. If you do carry on that risk is halved. The current advice is to carry on for four to six months after recovery unless there are pressing reasons to stop, such as side effects, illness or pregnancy.

Some people have repeated and long bouts of depression that can have a devastating effect on their lives and those of their family and friends. They may need to stay on antidepressants for much longer, perhaps several years. This is called maintenance treatment and is advised for someone who has had at least two previous bouts of serious depression in the last five years.

For advice on safe ways to stop taking antidepressants, see How to come off antidepressants safely