What is the difference between a medication that treats runny nose due to allergies, and a medication that treats runny nose due to a cold? Seems like allergies and colds have very similar symptoms.
There’s stuff that is pretty allergy specific (it targets the runny nose pathway that is occurring in allergy much more than in infection) – thats your antihistamines.
Then there’s stuff for general congestion – often marketed for colds, but works on all – these mostly work by shrinking blood vessels to reduce swelling and leaking that occurs with general inflammation- the nasal sprays are really prone to rebound stuffiness and shouldn’t be used for more than a couple of days.
The difference is usually just marketing. They’re generally the same meds and can be interchangeable.
They are also sometimes combined with different things. Allergy medications are usually just antihistamines. Cold and flu meds are usually antihistamines but frequently combined with a pain med or meds to treat chest or sinus congestion (but sometimes still just plain antihistamines)