Here is an answer I found on a different board from a veterinarian. Why do you want to give your dog valium? If it's for behavior issues, there are better ways to deal with that than drugging the dog.
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[Valium] is not recommended on a daily basis long term. Some dogs will have an adverse reaction to valium (diazepam) and behavior changes are possible. Diazepam is used in animals as a sedative, anti-convulsant, and to treat certain behavior disorders, such as anxiety. It may also be used for its effect of relaxing both skeletal muscles (muscles used in movement) and smooth muscles (muscles of the digestive system and urinary bladder). Use with extreme caution in animals that have liver disease or are aggressive. In some dogs it may cause a paradoxical behavioral reaction...suppressing anxiety it may release inhibitions and some normally aggressive dogs will become extremely aggressive. From a retired veterinarian with 30yrs experience.
I have one question why would you need Valium ??? Most dogs with the right amount of exercise a good diet and attention don't need drugs. I wonder is the drug for your convenience or the dogs?? I am not trying to be rude but like kids when I was young there were kids I was one of them who were hyper then was not ridyln it was called many hours of exercise and good food and routine. I hope this does not sound bad but maybe a long walk will help alleviate the problems of energy that the dog appears to have. If I am wrong let me know why you want the drugs