I am on Imbruvica and like many of us suffered from mouth sores for more then a year. I tried many remedies from prescription magic liquids against fungi to oral creams, L-lycine, Folic Acid and changing to organic tooth paste. All did not do the trick. At my CLL specialist's advice I tried Selenium, sold over the counter and 3 weeks after starting taking 1 pill a day the sores disappeared and I'm free of it for the past 2 months.
So, my friends, if you still struggle with this, try Selenium, give it a few weeks to work and please report back if it works for you. Good Luck!