Background:

Chronic pain is common and costly. Antidepressants are prescribed to reduce pain. However, there has not been a network meta-analysis examining all antidepressants across all chronic pain conditions, so effectiveness and safety for most antidepressants for pain conditions remain unknown.

Objective:

To assess the efficacy and safety of antidepressants for chronic pain (except headache) in adults. Our primary outcomes were as follows: substantial pain relief (50%), pain intensity, mood and adverse events. Our secondary outcomes were as follows: moderate pain relief (30%), physical function, sleep, quality of life, Patient Global Impression of Change, serious adverse events and withdrawal.

Design:

This was a systematic review with a network meta-analysis. We searched CENTRAL, MEDLINE, EMBASE, CINAHL, LILACS, AMED and PsycINFO databases for randomised controlled trials of antidepressants for chronic pain conditions up until 4 January 2022. The review was registered in PROSPERO (CRD42020171855), and the protocol was published in the Cochrane Library (https://doi.org/10.1002/14651858.CD014682).

Setting:

We analysed trials from all settings.

Participants:

We included trials in which participants had chronic pain, defined as longer than 3 months, from any condition excluding headache.

Interventions:

We included all antidepressants.

Main outcome measures:

Our primary outcome was substantial pain relief, defined as a reduction ˃ 50%. We also measured pain intensity, mood and adverse events. Secondary measures included moderate pain relief (above 30% reduction), physical function, sleep, quality of life, Global Impression of Change, serious adverse events, and withdrawal from trial.

Results:

We identified 176 studies with a total of 28,664 participants. Most studies were placebo-controlled (n = 83) and parallel armed (n = 141). The most common pain conditions examined were fibromyalgia (59 studies), neuropathic pain (49 studies) and musculoskeletal pain (40 studies). The average length of randomised controlled trials was 10 weeks. Most studies measured short-term outcomes only and excluded people with low mood and other mental health conditions.

Across efficacy outcomes, duloxetine was consistently the highest-ranked antidepressant with moderate- to high-certainty evidence. Standard dose was equally efficacious as high dose for the majority of outcomes. Milnacipran was often ranked as the next most efficacious antidepressant, although the certainty of evidence was lower than that for duloxetine. There was insufficient evidence to draw robust conclusions for the efficacy and safety of any other antidepressant for chronic pain.

Limitations:

The evidence for antidepressants other than duloxetine is poor. For duloxetine, it is not clear whether the effect applies to groups with both pain and low mood, since these groups were excluded from trials. There is also insufficient evidence on long-term outcomes and on adverse effects.

Conclusions:

There is only reliable evidence for duloxetine in the treatment of chronic pain. Duloxetine was moderately efficacious across all outcomes at standard dose. There is also promising evidence for milnacipran, although further high-quality research is needed to be confident in these conclusions. Data for all other antidepressants were of low certainty. However, the findings should not be read as an encouragement to prescribe antidepressants where other non-pharmacological intervention could be equally effective, especially in the absence of good evidence on side effects and safety.

Future work:

There is a need for large, methodologically sound trials testing the effectiveness of antidepressants for chronic pain. These trials should examine long-term outcomes (> 6 months) and include people with low mood. There should also be better reporting of adverse events, tolerance of drugs, and long-term compliance.

Study registration:

This study is registered as PROSPERO CRD42020171855.

Funding:

This award was funded by the National Institute for Health and Care Research (NIHR) Health Technology Assessment programme (NIHR award ref: NIHR128782) and is published in full in Health Technology Assessment; Vol. 28, No. 62. See the NIHR Funding and Awards website for further award information.

What was the question?:

Chronic pain is pain that lasts for more than 3 months. Over one-third of people across the world experience chronic pain. This often has a detrimental impact on people’s mood, disability and well-being. Antidepressants are often prescribed to reduce pain, but we are not sure which antidepressants work best for different types of pain, or whether they are safe.

We wanted to find out whether antidepressants were effective and safe for management of chronic pain.

What did we do?:

We searched for studies that had compared any antidepressant with any other treatment for any type of chronic pain (except headache). We compared all the treatments against each other using a statistical method called network meta-analysis. This method allows us to rank the treatments in order of best to worst for each outcome.

What did we find?:

We found 176 studies that included a total of 28,664 people with chronic pain.

Most of the studies (83/176) compared an antidepressant with a placebo (which looks like the real medicine but does not have any medicine in it).

The evidence from our analysis suggests that:

• Duloxetine is the antidepressant that we have the most confidence in. It was the best antidepressant for reducing pain and improving physical function.
• A standard dose of duloxetine was equally as effective for reducing pain as a high dose of duloxetine.
• Milnacipran was also effective at reducing pain, but we are not as confident in this result as in the one for duloxetine because there were fewer studies with fewer people involved.

Aside from duloxetine and milnacipran, we do not have confidence in the results from any other antidepressant included in this review, and even for duloxetine and milnacipran, we do not know the long-term effects.

It is important to recognise that the lack of evidence for the majority of antidepressants in this review does not necessarily equal a lack of benefit. Rather, this means that the large, high-quality trials required for us to be certain of an antidepressant’s effectiveness have not been undertaken.

Altogether, although duloxetine and milnacipran are effective, the results of this review should not be read as an encouragement to prescribe antidepressants where other non-pharmacological intervention could be equally effective, especially in the absence of good evidence on side effects and safety. These conclusions were informed by our patient and public involvement group.

Article history

The research reported in this issue of the journal was funded by the HTA programme as award number NIHR128782. The contractual start date was in April 2020. The draft manuscript began editorial review in April 2023 and was accepted for publication in March 2024. The authors have been wholly responsible for all data collection, analysis and interpretation, and for writing up their work. The HTA editors and publisher have tried to ensure the accuracy of the authors’ manuscript and would like to thank the reviewers for their constructive comments on the draft document. However, they do not accept liability for damages or losses arising from material published in this article.