Can you tell me when your dog may have eaten it? (How long ago)
How much does your dog weigh? What kind of dog is he/she?
How old is your dog?
Is your dog on any other medications?
Hello again and thanks for the extra information.
The good news is that this medication is actually used in dogs for narcolepsy and cataplexy, usually at a dose of 5-10mg/dog... or at the lowest, 0.25-0.5mg/kg dose... which for her size would be roughly 2-4mg... so if she ate a 2.5mg, she ate what would be the equivalent of a safe dose for her size.
Common side effects are usually signs of nervous system stimulation, such as hyperactivity, salivation, diarrhea, anorexia (loss of appetite), head bobbing, agitation, increased heart rate or respiration, tremors, and in severe cases, seizures. Dogs with underlying seizure disorders are at a greater risk.
If any of the above symptoms are noted, then tranquilizers such as valium can be given along with fluids to help speed the elimination of the drug from the system. Usually, within 24 hours, the medication has been cleared and no further side effects are seen.
Given that the possible amount ingested is actually a dose that would be given to a dog her size for underlying medical problems such as narcolepsy, side effects should be minimal and mild, if noted at all. If you are not sure what you are looking for, you could always have your vet examine her to be safe.
Best of luck to you.