if you want to eradicate fleas you do not go for the cheapest, you go to the vet and buy prevention for your dog and a spray for your house ... any flea stuff sold at walmart, petsmart, petco, etc., are a giant waste of money and time ... and no flea shampoo either ... flea shampoo is merely shampoo with harsh chemical added that have no residual effect and a soothing dog shampoo, with the dog left sudsy for at least 5 minutes, will kill all fleas on the dog effectively and without harm to the dog ... if you go budget you will be back posting "why do i still have fleas?" in no time and in the meantime your infestation will only grow ... for every one flea you see there are at least 30 more ... every adult flea lays 5-200 eggs a day which can take 2 days to 2 years to hatch ... i would also suggest buying a flea collar for your vacuum cleaner bag and vacuum every single day for 30 days to get up as many eggs and fleas as you can ... fleas do not live on the host (your dog, you), they live in the environment ... so it is important to treat both ... go cheap and your dog continues to suffer and the infestation is allowed to get out of control will not only require a professional exterminator, but your dog could suffer health problems from being infested, including anemia ...
Firstly : Dogs need DOG MEDICINE. Humans take human medicine. Our systems are waaaaaaaay different.
You really need to take the dog to the vet to find out what the cough is coming from. Your vet will get you the meds your dog needs.
Guaffinessin is a human medication (the medicine most commonly found in cough meds). "Chest congestion" (a human problem) may not even be the reason your dog is coughing.
You run a terrible risk of causing your dog more problems then good, and that's even IF the human medicine doesn't just kill the dog as some can because their system can't handle it.
Vet is best.