There are no scientific studies that I am aware of that say ANYTHING about the use of glucosamine, chondroitin, or MSM to prevent or treat dachshund disc disease. There ARE some well-designed studies showing benefit in humans as far as JOINT (not disc) problems like arthritis. There are even a couple of studies for dogs showing it helps with arthritis symptoms. It seems to improve the condition of the cartilage between the joints. Usually the combined supplements are used so it is hard to isolate which one is responsible. There is more data on the effectiveness of glucosamine than the other two (the other two can increase the price of the combos but there is less evidence they are effective).

I am very cautious when recommending supplements for several reasons (they are not regulated by the FDA so you may be buying anything from a plain sugar pill to a dangerous contaminant, they ARE drugs "natural" or not and can cause kidney and liver toxicity just like rx drugs, and they are not the magic bullets a lot pf people think they are). That being said, glucosamine is one supplement I do recommend to some of my chronic pain patients who have severe arthritis. There have not been a lot of reported toxicities with it (or chondroitin or MSM). I would consider it for a dog with arthritis. Using it to prevent or help disc disease in dachshunds is theoretical based on the fact that joint cartilage and discs contain some of the same components. However, my take on it is that if your dog has stiff joints, he is going to move and carry himself in a way that lessens the joint pain but might put abnormal stress on his back. If glucosamine can help maintain his joint fluidity, he will be able to move in a healthy, normal way and adapt to shocks when he jumps, etc - it might also keep him more active which will prevent weight gain that could further risk back injury, etc., etc.

Adding supplements to food (or treats) is usually just a way to get you to buy the food :D There is usually not enough of the supplement in there to make a bit of difference (so don't let that alone make you pay more for a food or treat - pick these based on quality of ingredients).

Vitamins are a different story... they are CHEAP so you could definitely get too many of those giving a fortified food and a multivitamin (same with people who eat Total cereal and Powerbars and take Centrum - too many vitamins). Too many vitamins CAN be toxic ranging from symptoms like diarrhea to neurological problems to increased risk of cancer. My vet suggested vit C and vit E. I won't give E myself, there were too many studies saying it was a "miracle vitamin" for cancer and heart disease that were stopped early because people were dying faster when they took vit E. As far as the C, it is water-soluble (instead of fat-soluble) so for the most part I think the body will get rid of what it doesn't use. But I still think the back disease prevention is more theoretical and based on testimonials. Even the role of vit C in immunity is not conclusive - for every study that says it prevents colds or improves wound healing, there is one that finds it does absolutely nothing.

If you want to use an herbal or supplement, I always recommend www.consumerlab.com which does independent testing of these products to see if they contain what the label says they do and that they are free of impurities - at least then you know you are getting your money's worth and giving what you want your dog to have . You can get some info free on the site but you need a subscription to access the full data (I do subscribe if anyone ever wants me to research something). Of course they are usually researching the human products (which dogs can use) but in the case of gluc/chon they did actually review a couple veterinary products.