My dentist never told me this & I struggled with root canal problems for years. After the 5th treatment with Antibiotics I started searching and came across a thread claiming Zinc lozenges might help...
Mind you, they have to be lozenges so they will dissolve in your mouth letting the mucous membranes absorb the zinc into the tissue. So NOT regular zinc tablets.
Not sure if it will work in a late stage of infection, but for me, as soon as I feel tooth ache coming on, I try a lozenge and for 10 years have not had root canal problems ever since (knocks on wood).