Amitriptyline; Why are we prescribed an anti-depressant for chronic pain?

There are 16 million back pain sufferers in the UK, and on any given day 2.5 million of those will be in pain. Pain relief in the form of analgesics are often prescribed by your GP in the more severe cases. Some of these pain killers have several uses and amitriptyline often causes alarm when people discover it is also used as an ant-depressant.

Amitriptyline is from a class of drugs called tricyclic antidepressants (TCAs). It was originally developed to combat anxiety and depression as far back as the 1930’s. In the 1980’s however it was discovered that amitriptyline also had a secondary action in terms of alleviating chronic pain, and since then this has become its primary purpose. Many of our patients have an inbuilt fear of antidepressants and they naturally become alarmed when the discover they are taking a TCA for the purposes of managing chronic pain. These concerns are well summarized in this statement;

‘It’s hard to get people to take antidepressants for chronic pain conditions because of what they stand for; they just tell you they’re not depressed,’ says Dr Peter Skew, President of the British Institute of Musculoskeletal Medicine. ‘We’ve got to get across the idea that medicines can do different things’.

So why are you taking a TCA for pain relief and does this expose you to any adverse risks?

Although amitriptyline was originally devised to manage anxiety it was observable that amitriptyline influenced mood and depression. Subsequent research revealed that amitriptyline had an effect on certain neurotransmitters by influencing their production and efficacy. One of these is serotonin which has an important role in mood, emotion and pain regulation. Another is norepinephrine which has the general function of  mobilizing the brain and body for action. The precise mechanism of how TCA’s work is not fully understood but it is known that amitriptyline blocks chemical receptors (or docking sites) that these neurotransmitters utilise.This stimulates an increase in production of these transmitters which then helps shut ‘pain gateways’ and increase your pain threshold. Using this mechanism amitriptyline helps alleviate pain from conditions like arthritis, migraine, shingles and MS.

It is important to remember that is only effective for neuropathic pain. This is associated with long term chronic pain when changes have taken place in the spinal cord because of long term exposure to nerve-ending stimulation. These damaged nerve endings will respond to amitriptyline. The important point here is that amitriptyline will not help with burns or acute injury.

To work as an antidepressant, amitriptyline needs to be taken in doses of up to 150mg a day. For pain relief it is started at a low dose (10 or 25 mg a day) and gradually increased in 10 or 25 mg increments each week up towards 75 mg if any side effects are tolerable. Pain is likely to improve over a 2-8 week period. Trials have shown that patients get pain relief before any change of mood at such low levels

In terms of side effects patients taking amitriptyline may experience drowsiness, a dry mouth and constipation (10-15% of people) although these should decrease as you carry on taking it and your tolerance increases. Depending on age, these may be severe enough to make the whole experience unpalatable, but in most cases any side effects are relatively minor and confined to the initial few days or weeks.

Amitriptyline is not addictive but if discontinued, it should be withdrawn slowly over two to three weeks in order to avoid withdrawal symptoms of headache and malaise.

A full list of the side effects is detailed below

Very common: may affect more than 1 in 10 people

• sleepiness/drowsiness

• shakiness of hands or other body parts

• dizziness, headache, dizziness when you stand up due

to low blood pressure (orthostatic hypotension) • irregular, hard, or rapid heartbeat

• dry mouth, constipation, nausea

• excessive sweating

• weight gain

• slurred or slow speech • aggression

• congested nose.

Common: may affect up to 1 in 10 people

• confusion, disturbance in attention, disturbed

coordination, agitation

• sexual disturbances (decreased sex-drive, problems

with erection)

• changes in taste, feeling thirsty

• numbness or tingling in the arms or legs

• dilated pupils

• heart block

• fatigue

• low sodium concentration in the blood

• urination disorders.

Uncommon: may affect up to 1 in 100 people

• excitement, anxiety, difficulties sleeping, nightmares • convulsions

• tinnitus

• increased blood pressure

• diarrhoea, vomiting

• skin rash, nettle rash (urticarial), swelling of the face

and tongue

• difficulties passing urine

• increased production of breast milk or breast milk

out ow without breast feeding

• increased pressure in the eye ball

• collapse conditions

• worsening of cardiac failure

• liver function impairment (e.g. cholestatic liver disease).