My family's lab x golden mix has had joint problems since about 6 months old. He has very bad loose hips and also his knee would pop out of the joint while he was running around. He was first given some kind of injections by the vet regularly for a few weeks and then has been on Glyco-Flex pretty much his entire life. I think that intervention avoided surgery. Now he's about 9 years old and still very mobile and playful.
I personally will not give a dog any vitamin that is not a water soluable vitamin without getting a CBC done. If the blood work then shows that they are lacking something. Vitamin C is very helpful in the stesses associated with growing puppies. And it is water soluable. Any thing that the excess can not be eliminated in the urine, can build up in the body and cause harm. I do think the joint suppliments they suggested are also safe to give. But I think I would want a confirmed diagnosis as to the reason for giving it. But that's just me. Is this something you are thinking about for a Mastiff? And what is the age of the dog? And what are you currently feeding?
He's your friend,your partner,your defender your dog.You are his life,his love,his leader. He will be yours faithful and true to the last beat of his heart.You owe it to him to worthy of such devotion