Although chondroitin and glucosamine are often lumped into the same category, they're quite different.

Chondroitin is part of a large protein molecule (proteoglycan) known to give cartilage its elasticity. It's found in all human and animal cartilage and is an important part of the repair and maintenance cycle in cartilage.

Arthritis occurs when cartilage has become worn down or inflamed (or both), leading to pain, stiffness and loss of function in the affected joint. Most treatments aim to reduce the inflammation and control the pain of inflammation. Chondroitin is thought to work by helping repair cartilage.

The chondroitin in commercial preparations is chondroitin sulphate and is made from animal cartilage, such as wind pipes or shark cartilage. If you're vegetarian or you don't use animal products, then this isn't for you.

There have been a number of studies assessing whether chondroitin - either alone or with glucosamine - is an effective way of reducing the pain of arthritis. So far, the conclusion has been that it's at least as good as the standard medical therapies such as ibuprofen and aspirin.

There have also been studies suggesting chondroitin can reduce inflammation, which is based on measurements of the joint space in knees. In several studies, the knees showed improvement. However, these studies have been criticised on technical grounds because they were based on X-rays that were taken at an angle that may not give correct information.

So, those studies are now being repeated by the US National Institute of Health, with a huge trial of both glucosamine and chondroitin.

This doesn't mean that products claiming to be cures for arthritis or good for arthritis are as good as they say.

But what both glucosamine and chondroitin show is that when something comes from the natural therapy or supplements industry and consumers find it effective, even conservative medical scientists will take it seriously.

When they've done the tests and, perhaps, come up with some evidence of useful effect, doctors will offer it to patients.

There is, however, a catch. If a treatment is classified as a supplement, the rules about purity, delivery, testing and so on are different from those classified as drugs.

In other words, you may have a great substance that does all the things you want. But if it comes in a form that stops it from being active or has salts or impurities that affect other components of your system - such as stimulating allergies - then the product may not only be ineffective, it may even be harmful.

Worse, you may take one form and find that a particular dose is working well for you. The next time you buy it, the dose doesn't have the same effect. It may be stronger or weaker. This is because the dose isn't constant in some preparations because of the production process.

So, whenever you have something that works well, stick with that version.