Because most of the contents were gone I will figure as if he ate most/all of the ingredients.
We see toxicity symptoms in dogs at a dose of Acetaminophen higher then 75 mg per kilogram (2.2 pounds per kilogram) of body weight.
At 8 pounds he is 3.64 kilograms, so he would need to eat 272mgs of acetaminophen. (3.64kg X 75mg/kg = 272mg).
Unfortunately the amount he ate will be very close, and possibly enough to be toxic. Acetaminophen causes red blood cell damage because it binds to hemoglobin in the red cell, and liver damage. Signs to watch for with acetaminophen toxicity would be brown or gray colored gums, difficulty breathing, edema or swelling of the face, neck, legs or feet, vomiting, yellow color to the skin and in very severe cases coma.
Dextromethorphan is a cough suppressant we can use in dogs. We do give this to dogs at a dose of 0.25mg to 0.5mgs per pound of body weight. An 8 pound dog would take 2mgs to 4mgs every 8-12 hours, so 10mgs is fairly high, although this drug is unlikely to do much more than make him very sleepy.
Finally Phenylephrine is a decongestant. This one is a little worrisome because dogs handle decongestants poorly, but it is a small amount and they also don't absorb them well from the gastrointestinal tract, I would more concerned if this were a nasal spray.
Let me check the toxic dose for this one.
Phenylephrine is a sympathomimetic amine that is used as a decongestant and to treat hypotension (low blood pressure). It is poorly absorbed orally in dogs in the tablet form, because it is so extensively metabolized by the intestines and liver, so that is good news for your girl.
The nasal spray preparations are much more toxic as they are absorbed much, much better directly across mucous membranes (gums). The drug is absorbed quickly such that peak effects are seen in 30 minutes, and it is essentially out of the system in 6 hours or so.
The most common signs after tablet ingestion in dogs include vomiting, hyperactivity, lethargy, panting, and trembling. Toxic dosages in these pups ranged from 0.23 to 30 mg/kg. Hypertension was not a problem as it is in people that overdose with this drug.
At 8 pounds (3.64kgs), if he ingested 5mgs this may cause some symptoms as the minimal symptom threshold at his weight is 0.84mgs (3.64kg X 0.23mgs/kg = 0.84mgs) you may see symptoms including some panting and mild nausea. But because it is so poorly absorbed he may be just fine. The sedative effects of the dextromethorphan may balance the stimulant effects of the decongestant.
At this point I am most concerned about the amount of Acetaminophen he ingested.
It is well worth getting him to an emergency clinic for treatment for that toxicity including giving activated charcoal, fluids and a glutathione source (S-adenosyl-methionine or SAMe), as well as an antidote.
Please let me know if you have any further questions.