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Dogs who will not put weight on a leg could have a dislocated hip, or knee injury such as an ACL tear. Often a dog will allow the owner to move the leg around since it is most painful when they attempt to move it on their own power. You can read about dislocated hips here:
And read about ACL injuries here:
If he is actually putting some weight on the leg, then it may just be a bruised leg or soft tissue injury like a sprain. For this I would recommend strict cage rest for a few days and then reevaluate the situation. If there is some improvement, you might continue crate rest for a few more days and then progress to leashed walks. A partial tear of an ACL can be reinjured and tear completely with too much exercise, so keeping the dog calm is recommended until your vet can evaluate the injury.
Buffered aspirin can be given to a dog with a dosage of up to 5-10 mg per pound every 12 hours for pain.. Keep in mind that a dog's body does not metabolize aspirin in the same way as a human and thus should not be given more than a day or two without contacting your Vet. The aspirin may need to clear your dog’s system before other medications can be given, so keep that in mind if you decide to give aspirin and be sure and tell your vet when your dog is seen. It can take a week or more. Read side effects and precautions here.
Since it is a holiday weekend, you would need to have him seen first thing in the morning or wait until Monday unless you want to incur an ER charge.
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