I have a thin red rash under my bottom lip. how should i treat it? i used to get it when i ate ketchup and a couple years back when i used to smoke but it always went away. now it won't.

Have it checked out: The association of ketchup and smoking with a rash on the face sounds like an irritant or allergic contact dermatitis. Some over the counter 1% Hydrocortisone should help. If this does not, you should have it evaluated by your doctor. Sometimes, what appears to you to be a reaction to something can in fact be something more serious (like precancerous skin changes or actinic keratosis).

Have it evaluated: It's not possible to make a diagnosis or specific recommendations without seeing you. However, a rash which won't clear up is not normal and could represent a skin cancer. It most likely can be treated in either case. I would make sure you mention it to your pcp and or make an appointment with a dermatologist.