Get Your Dog Care Questions Answered by Experts
It really depends on what is causing it.
I suggest that you wash the dog well with antibacterial soap in that area to remove any debris or residue that might be causing a contact allergy.
This might be all that it takes to cure it.
If not, your dog might have a bacterial or fungal infection or yeast that is causing the redness.
This could also be an allergy. Dogs can have allergies to fleas, products, grass, etc. If this is the case, your vet can prescribe a dose of benadryl that will help.
Let me know if you have more questions.