Coal tar shampoo can alleviate skin and coat problems like overly oily coats, itchy skin and dandruff. The special wash doesn't have too many side effects, but it's not something you should use without first talking to your vet.
Coal tar shampoo is fairly safe for your pup. At worst, veterinarian Jeff Vidt notes it can cause irritation and dryness, exacerbating the skin problems it was supposed to cure. While that's not a side effect you probably want to encounter, it's not a serious one. This is assuming you use a coal tar shampoo meant for canines. Human shampoos can cause severe skin problems because they aren't made with a dog's skin pH in mind.
Consult Your Vet
Avoid purchasing a coal tar shampoo until you talk to your vet. Plenty of conditions can cause greasy coats or flaky, irritated and dry skin. Some of these conditions, such as allergies, infections and ringworm, require treatment and medications aside from lathering on tar shampoo. Even if coal tar shampoo can treat your pup's condition, your vet may want you to use it more frequently than you would a normal shampoo -- such as once or twice weekly until the dog's skin heals.