I have an 8 year old dachshund that has been diagnosed with back problems. I have heard that vitamin C is great, but I'm not sure how much to give her? Can anyone help?

I did a search, notdawn, and found this website Animal Health Care - Your Pet Health and Pet Care Information Destination that will help you with a lot more information then you asked for. Basically they say that dogs produce their own Vitamin C so you won't find a dosage factor anywhere. May I suggest you figure out the weight ratio with your own (may be difficult but can be done). I have found my vet recommendations are accompanied by a dosage for the weight of the dog.

You may also want to give your pet glucosamine/chondroitin which is good for osteoarthritis but my dog was taking it for ligature strength; she is a very large 60 lb. dog and benefited greatly in many ways from this supplement. I take it as well.

Unfortunately your dachshund is suffering from a problem in longer dogs with short legs.

Vitamin C and D are naturally produced in a dogs system so unless your vet says that in your dogs older years he is not producing the proper amount I would try something different.
You might want to try Hylaronic Acid. Vets give it to animals for their joints and tendons.
I take it myself and notice a world of difference.

Where did you hear vitamin C is good for back problems? Was there any proof of it, or was it just speculation or at most some anecdotal evidence? The truth is, any decent dog chow has all the vitamins a dog needs. The best thing you might be able to do is make sure the back isn't suffering from any extra stress due to being overweight.

Evaluate the dog as illustrated in this link, LongLiveYourDog.com - Life Span Study - Rate Your Dog You may want the vet to confirm your judgment. Adjust the dogs food and exercise as needed to reach its ideal body condition.

I would talk to my vet before I started medicating my dog with untested and unproven remedies.