i would say to do revision in small bursts with plenty of breaks, say do 25 minutes and then a 5 minute break and repeat or try even less find what time you can concentrate for solidly and work with that trying to do long stretches of revision will be working against your adhd. ( i dont have it myself but i have dyslexia and dyspraxia so guess my concetration isnt quite normal and plenty of breaks helps me)
Also try doing more fun revision say do practical things like flashcards and quizlet is a brilliant website.
How long have you been on your medicine, if its a while and its not working so well i d say go ask your gp if they ll consider switching you to something else?