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Written by Helen Marshall, pharmacist
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Antidepressants usually help mood improve over a number of weeks
or months.
There are a few antidepressants that don’t fit neatly into the
tricyclic, SSRI or MAOI categories, but which work in similar ways to one or
more of these groups. They are:
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duloxetine
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mianserin
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mirtazapine
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reboxetine
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trazodone
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tryptophan
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venlafaxine.
How do they work?
It is thought that depression may be linked to an imbalance of
chemicals in the brain. Within the brain there are chemical messengers
called neurotransmitters. Examples of these are noradrenaline and serotonin.
Neurotransmitters are involved in controlling or regulating bodily functions,
and noradrenaline and serotonin are involved in the control and regulation of
mood.
When these chemicals are released from nerve cells they act to
lighten mood. When they are reabsorbed into the nerve cells, they no longer
have an effect on mood. When depression occurs, there may be a decrease in the
amount of these neurotransmitters released from nerve cells in the brain.
All these other antidepressants work, through slightly different
mechanisms, by increasing serotonin and/or noradrenaline in the brain. This
alters the imbalance of chemicals that is thought to be important in causing
depression and hence relieves the depression.
Mianserin and trazodone are related to the tricyclic antidepressants. Mianserin works by preventing the re-absorption of noradrenaline back into
the nerve cells in the brain. It also enhances a small degree of serotonin
activity.
Trazodone works by preventing the re-absorption of serotonin
back into the nerve cells in the brain. It may also act to mimic the
mood-lightening effect of released serotonin and enhance a very small degree of
noradrenaline release from nerve cells in the brain.
Venlafaxine and duloxetine are called serotonin and noradrenaline reuptake
inhibitors (SNRIs). They work by preventing both serotonin and noradrenaline from
being reabsorbed back into the nerve cells in the brain.
Reboxetine is called a noradrenaline reuptake inhibitor (NARI).
It works by preventing the re-absorption of noradrenaline back into the nerve
cells, resulting in an increase in the amount of noradrenaline in the brain.
Mirtazapine is called a noradrenergic and specific serotonergic
antidepressant (NaSSA). It works by blocking receptors called alpha-2 receptors
that are found on nerve cells in the brain. This enhances the action of
noradrenaline and serotonin in the brain.
Tryptophan is an amino acid that the body needs to make
serotonin. It increases serotonin levels in the brain.
How long do they take to work?
Antidepressants can take a while to have an effect, so you may
not feel better immediately when you start treatment with one. You may
experience an effect on your mood within two weeks, however, the full benefits
of treatment may not occur for a further two to four weeks. If you feel your
depression has got worse, or if you have any distressing thoughts or feelings
in these first few weeks, then you should talk to your doctor.
Venlafaxine may start to work slightly quicker than other
antidepressants.
How long will I have to take them for?
Antidepressants usually help mood improve over a number of weeks
or months. Even when things seem back to normal, you should keep taking them
for at least a further six months to minimise the chances of the depression
coming back.
Are they addictive?
No. It is possible for antidepressants to produce unpleasant
withdrawal symptoms (sometimes called a discontinuation syndrome) when they are
stopped. But this is temporary, does not involve a craving for the medication,
and can usually be avoided if the drug is tapered off rather than stopped
suddenly. This is not addiction.
Withdrawal symptoms can sometimes occur if you miss a dose of
the antidepressant, which is why it is important to take them as directed by
your doctor.
When stopping treatment, withdrawal symptoms can be minimised or
avoided entirely if the dose of the antidepressant is gradually decreased over a period of
a few weeks. Your doctor will help you do this.
Which one is best for me?
All these antidepressants will usually be second-line options
for treating depression, when an SSRI or tricyclic has been ineffective.
The choice will depend on your individual features, such as
other conditions you have, whether you have co-existing anxiety, or other
medicines you may be taking, together with the individual characteristics of
each of the drugs.
For example, mianserin and trazodone have sedative properties
and may be useful for people who are also agitated or anxious.
Mirtazapine has a lower incidence of side effects compared with
TCAs, SSRIs and other antidepressants, so there is less dry mouth,
constipation, nausea, vomiting and sexual dysfunction. However, the main side
effects seen with mirtazapine are drowsiness, blurred vision and increased
appetite that can result in weight gain.
Reboxetine may also have fewer side effects than SSRIs or
tricyclics, but it is one of the newer antidepressants and there is limited
information available on it compared with other antidepressants.
Venlafaxine is useful for depression associated with anxiety
and can also be used to treat generalised anxiety disorder. It has similar side
effects to the SSRIs. It should not be prescribed to people with an irregular heart beat or high blood pressure and should be used with caution in heart
disease.
Duloxetine is the newest of these antidepressants and can also be used to treat diabetic nerve pain.
You can read more detailed information about these
antidepressants in the factsheets linked below:
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Cymbalta (duloxetine)
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Edronax (reboxetine)
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Efexor (venlafaxine)
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Efexor XL (venlafaxine)
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Mianserin
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Molipaxin (trazodone)
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Optimax (tryptophan)
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Zispin
(mirtazapine)
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Last updated 27.03.2007
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