From the Guidelines

Magnesium can be used for pain management, but its effectiveness is limited and dependent on the method of administration.

Intra-operative and postoperative i.v. magnesium sulphate has shown no significant benefit in pain management compared to placebo 1.
Topical magnesium sulphate has shown a transient effect on pain management, but its duration is limited 1.
Oral postoperative magnesium sulphate has been reported to reduce pain scores when compared to metamizol, but more research is needed to confirm this finding 1.
Local administration of magnesium sulphate has shown a small effect on pain management, but the evidence is not strong enough to support its widespread use 1.

Magnesium sulphate has been compared to other therapies such as antibiotics, sucralfate, and dietary protocols, but its effectiveness is still uncertain 1.
Sucralfate has shown a weak analgesic benefit when used as an adjuvant with repeated applications over several days, but its effectiveness is limited 1.
Dietary protocols such as restricting postoperative diet to liquids or cold diet have not demonstrated any significant benefit in pain management, but minimizing fasting time and using ice lolly have shown some promise 1.

From the Research

Magnesium is commonly used in clinical practice for acute and chronic pain, and has been reported to reduce pain intensity and analgesics consumption in several studies 2, 3, 4.
The level of evidence for the efficacy of magnesium in reducing pain and analgesics consumption is globally modest, and more studies are needed to achieve a sufficient level of evidence 2, 3.
Magnesium has been shown to produce an antinociceptive effect on animal models of neuropathic and inflammatory pain, and has also been shown to exert an analgesic effect on humans in conditions presenting acute and chronic pain 5, 4.
The primary mechanism through which magnesium produces its analgesic effect is believed to be blockade of the N-methyl-D-aspartate (NMDA) receptor ion channel in the spinal cord, as well as modulation of potassium and calcium channels 5, 4.

Post-operative pain: magnesium has been shown to reduce pain intensity and analgesics consumption in post-operative pain 2.
Migraine: magnesium has been shown to reduce pain intensity and frequency of migraines 2, 3.
Neuropathic pain: magnesium has been shown to produce an antinociceptive effect on animal models of neuropathic pain, and has also been shown to exert an analgesic effect on humans with neuropathic pain 3, 5.
Chronic pain: magnesium has been used for the treatment of chronic pain, although the evidence is still limited and more studies are needed to determine its efficacy 3, 4.

The safety of magnesium for pain management is still not well established, as adverse events are inconsistently reported in studies 3.
The efficacy of magnesium for pain management is still equivocal, although some studies have reported positive results 2, 3, 5, 4.
More studies are needed to determine the optimal dosage, formulation, and route of administration of magnesium for pain management, as well as to establish its safety and efficacy 2, 3, 4.