It depends. Hay fever and colds r 2 different diseases. Some ingredients treating one disease is not needed in the other. Otc cold meds often contain tylenol, (acetaminophen) or ibuprofen. Since allergic patients don't have fever, they don't need these two meds. Tylenol (acetaminophen) can cause liver damage if taken too much, esp in people already have liver disease. Ibuprofen can cause stomach pain, sometime ulcer. Be careful.
Maybe. I actually don't usually recommend antihistamines for colds. In general, lots of fluids and rest are what the body needs...As your immune system does the job of fighting the cold.