Have constant phlegm in back throat can it be oral thrush?

Not likely: In the absence of immune conditions (hiv, poorly controlled diabetes, cancer, etc.) or recent treatment with antibiotics, thrush is pretty rare in the 5 to 70 year age group. Phlegm or mucus in the throat is extremely common. Common causes include allergy/ sinus drainage, dehydration, and reflux. These common conditions are evaluated by ENT physicians, often with camera exam in the office.

Sinus discharge: Without a clinical examination it is difficult nto tell what your problem is but typically a mucus discharge down the back of the throat is more typically related to sinus issues. Oral candidiasis (thrush) more typically presents a red or white patches in the on the cheeks, tongue, and back of the throat. There is typically not a discharge associated with thrush. Please see your physician.