DWI in California question?

i was given a dwi for being under the influence of marijuana. i am prescribed marijuana for adhd and i had taken a dose of medication about 2 hours before driving. now in my opinion after 2 hours i am no longer under the influence. i got stopped at a dui checkpoint and they found marijuana on my person and in my car. after giving me a good half hour of tests the officer determined i was under the influence of marijuana. i believe that he thinks this merely because i have my prescription on me and marijuana in my car i do not think that he correctly evaluated me in the tests so my question is, is there anyway i can fight this