My 3-month puppy can't stand on her back legs. What medicine?

Poor Lucy! It's very concerning that a three month old puppy is not able to stand on her back leg. She could have severely injured her leg such as breaking a bone or tearing a ligament making her unable to use it. Unfortunately it's illegal for the experts on Pet Coach to prescribe or recommend any medications. That's because doing so requires a veterinary client patient relationship which cannot be achieved via an internet forum. In addition, the experts cannot say for certain if a specific drug is safe for your dog. There is also no way I could say what treatment would be best for her without seeing her in person and diagnosing the cause of her problem. As I said, it's very concerning when such a young puppy won't use her rear leg, and because of that, I recommend an immediate vet visit. Please take her into a vet now. Best of luck.