The inside of my mouth is constantly itchy. what could this be?

Allergies maybe: You might have what is call oral allergy symptoms. Usually cause by raw fruits or even seasonal allergies. If concerned u could have ur doctor run some tests

Probable allergy: As Dr. Tran mentioned, the most likely cause is allergy. I used to get this from raw apple skin- only the skin, and if cooked was fine, but also occ. get it from pollens. After avoiding raw apple skin for many years the problem went away. But the unusual thing is your mentioning it's constant- maybe it's a food you eat daily. Consider seeing an allergy specialist or ENT doc.