It is claimed that magnetic therapy has been used for thousands of years by ancient Greek, Egyptian and Chinese civilisations. It is possible to obtain a wide range of products designed for animals which claim to use magnets to improve mobility and decrease pain in arthritic animals. Some products are claimed to treat a wide range of conditions. Products available include collars and blankets for small animals and boots or rugs for horses.
Magnets are supposed to work by subjecting the cells to an external magnetic field which improves circulation, maintains the body's optimal pH and maintains electrical conductivity in the cells. All this, it is claimed, increases the body's ability to heal itself. That the magnetic field would have an effect of some kind on cells is not unlikely because so many molecules within the cell are electrically charged. Just how this effect works and why it helps healing and pain relief is something which requires further investigation. Perhaps it may work a little bit like a TENS machine which is certainly very effective at relieving pain? It is also unclear what type and strength of magnet should be used with different companies seeming to promote different types of magnet.
I have no idea why these products should work but there is plenty of anecdotal evidence to suggest that they do!
There is some published scientific evidence which suggests the effects of magnetic therapies in patients with pain from musculoskeletal problems were positive. For example a study published in the BMJ suggested that there was a decrease in osteoarthritic pain in human patients wearing magnetic bracelets compared with those who did not. (It was unclear whether this response was due to the placebo effect of wearing a bracelet or not.) (Harlow et al; 2004)
My own view on the use of magnetic collars and similar products is that they are worth a try. I would recommend them mostly for animals with osteoarthritic pain or some other musculoskeletal pain such as sprains or strains. If they do work they would certainly be safer and cheaper than steroidal or non steroidal anti-inflammatory drugs. If they don't work for your pet then they are unlikely to do any harm.
Harlow, Greaves, White, Brown, Hart and Ernst: Randomised controlled trial of magnetic bracelets for relieving pain in osteoarthritis of the hip and knee. BMJ 2004;329 : 1450 - 1454