How long ago did your dog eat this?
How is he currently acting?
How did he acquire this medication? Were any medications given in conjunction with this?
Ok, so this was 10mg phenylephrine then. The best option would be to induce vomiting now before the medication can affect her. You can do this by giving her 3 tablespoons at a time of hydrogen peroxide until she vomits. Keep giving it to her until she does.
There are some issues with decongestants and dogs. They do not process the medication well and they can cause hypersalivation, vomiting, hyperactivity, seizures, hypertention, and cardiac arrythmia's. So, it would be best to induce vomiting.
However, the dose is low and most dogs do not react adversely to this drug when given as a single dose. The potential is there and if there is underlying illness that we are not aware of this could make things worse. If you notice any excessive panting, irritability, or sudden weakness then a trip to the emergency vet would be in order.