If you have an optional state issued ID card, he can't arrest you. If you just have your written recommendation, it would be up to the officer because these cannot be verified right away. If arrested, the recommendation should get your charges dropped in court after it has been verified with the judge, but it won't always prevent an arrest. Some local cops have policies on how to deal with medical marijuana patients, some don't, so sometimes it just depends on the officer's attitude. California Highway Patrol does have a policy to not arrest medical marijuana patients or confiscate their medicine, as long as you have your original written recommendation and are within the legal limits.

A good general rule of thumb is to not advertise your medical marijuana status. It's never a good idea to tell a cop you are a medical marijuana patient unless they need to know. If you are asking because you just wanted to show off your medical marijuana status to a cop, think again. If you are asking because you are genuinely concerned with arrest as a legal medical patient, you shouldn't have any problems as long as you are smart with your medicine.

Just remember, driving while stoned is always a DUI, medical patient or not.