There is very low quality evidence that morphine taken by mouth has any important effect on pain in people with moderate or severe neuropathic pain.
Neuropathic pain comes from damaged nerves. It is different from pain messages that are carried along healthy nerves from damaged tissue (a fall or cut, or arthritic knee). Neuropathic pain is often treated by different medicines (drugs) to those used for pain from damaged tissue, which we often think of as painkillers. Medicines that are sometimes used to treat depression or epilepsy can be effective in some people with neuropathic pain. Opioid painkillers are sometimes used to treat neuropathic pain.
Opioid painkillers are drugs like morphine. Morphine is derived from plants or synthesised by chemists. Morphine is widely available for use as a painkiller, usually given by mouth.
Our definition of a good result was someone with a high level of pain relief and able to keep taking the medicine without side effects making them want to stop.
In February 2017, we searched for clinical trials in which morphine was used to treat neuropathic pain in adults. Five studies satisfied the inclusion criteria, randomising 236 participants to treatment with morphine, placebo, or other drugs. Studies lasted four to seven weeks. Few studies reported beneficial outcomes that would be regarded as clinically relevant.
Four small studies reported that pain was reduced by between a quarter and a third in some people. This level of pain reduction was experienced by 6 in 10 participants with morphine and 4 in 10 with placebo. Between 1 and 2 in 10 participants withdrew from treatment with both morphine and placebo, but the reasons were not given. Side effects were poorly reported, but were more common with morphine than with placebo, and included drowsiness, dizziness, constipation, feeling sick, dry mouth, and decreased appetite.
Quality of the evidence
The evidence was of very low quality. This means that the research did not provide a reliable indication of the likely effect, and the likelihood that the effect will be substantially different is very high. Small studies like those in this review tend to overestimate results of treatment compared to the effects found in larger, better designed studies. There were other problems that might lead to over-optimistic results. The very low quality evidence and the lack of any important benefit mean that we need new, longer-lasting, large trials before we will know if morphine is useful for the treatment of neuropathic pain.