Hello, my name is***** and I have over 20 years of experience as a veterinarian. I understand that you are concerned about Gracie ingesting 5mgs of Valium (diazepam).
The good news is that she isn't a tiny dog and that we do use this drug in dogs to relive anxiety and help control seizures. Of course Gracie didn't need this drug so ideally she wouldn't have taken it, but this shouldn't be a horrible mistake.
A usual low dose to help with anxiety ranges between 0.12 mg per pound of body weight up to 1 mg per pound of body weight given orally once per day as needed. That means a 37 pound could take 4mgs to 37mgs relatively safely. A 5mg dose is pretty low for a pup her size.
If Gracie is otherwise healthy I would not expect her to suffer any long term effects from this.
Because this drug is metabolized by the liver if she had liver disease I would expect that even a small dose would be more likely to affect her, and it would last longer than usual.
Things you might notice include sleepiness, or paradoxical hyperactivity, incoordination, increased appetite, and with very high doses extreme sedation, coma, low blood pressure and loss of reflexes.
At the dose she ate I would be surprised to see more than very mild sedation and clumsiness. These symptoms should not last more than 12 hours.
As a precaution keep her away from stairs and supervise her closely today, but she should be fine.
Please let me know if you have any further questions.