Hi again, and thanks for your additional information.
The following is the dosage recommendations for Xanax in dogs, and possible side effects and interactions with other medications; I'm assuming your dog is not taking any other medications at this time. (from: http://www.petplace.com/drug-library/alprazolam-xanax/page1.aspx)
# XXXXX typical dose range for alprazolam in dogs is 0.01 to 0.05 mg per pound (0.02 to 0.1 mg/kg) given orally every 12 hours.
# XXXXX cats, the dose is 0.125 to 0.25 mg per cat as needed up to every 12 hours.
# XXXXX duration of administration depends on the condition being treated, response to the medication and the development of any adverse effects. Be certain to complete the prescription unless specifically directed by your veterinarian.
# XXXXX and Side Effects
# XXXXX generally safe and effective when prescribed by a veterinarian, alprazolam may cause side effects in some animals.
# XXXXX should not be used in animals with known hypersensitivity or allergy to the drug.
# XXXXX should also be avoided in patients with muscular weakness or glaucoma. Alprazolam should be used with caution in animals with liver disease.
# XXXXX may interact with other medications. Consult your veterinarian to determine if other drugs your pet is receiving could interact with alprazolam. Such drugs include other central nervous system depressants, digoxin, phenytoin and theophylline.
# XXXXX most common side effect is excessive sedation and loss of motor control, but these effects occur at doses greater than those needed for its anxiety-reducing effect.
# XXXXX some animals, alprazolam may cause a paradoxical reaction, and may cause excitement or worsen aggression.
# XXXXX treatment with alprazolam can lead to physical dependence, which can result in undesirable behavior changes if the drug is abruptly discontinued.
If the normal dosage for a dog who is prescribed the medication is .01-.05 per pound, 15lbs X .05=
.75, so if he ate one, it should just make him sleepy; if you think he ate 2, I'd have him evaluated by the ER vet to make sure it doesn't depress his system too much. Also, as you could see, sometimes it has the opposite effect, and can cause excitability.
You also have the option of inducing vomiting, which you can do by giving him 1 teaspoon of 3% hydrogen peroxide on the back of the tongue; if he doesn't throw up in 20-30 min., you can repeat this dosage once more. Walk him outside to help move his stomach contents around and if/when he throws up, it should be there (outside), unless it's too cold. If you'd rather the vet induce vomiting, you have that option.
I think he'll be fine, just very sleepy. Monitor him closely, and if you feel he's behaving weirdly, bring him to the ER vet center, immediately.