If the officer has already found your medical marijuana, showing your recommendation is a good idea. Your medical marijuana recommendation will provide you a limited protection against prosecution in court pending the amount in possession. If you are found in possession a greater amount of medical marijuana than allotted by California law, your recommendation will not necessarily protect you beyond the legal possession amounts. Under California law, unfortunately, there is no hard and fast rule for legal possession amounts. The technical answer is an amount reasonably related to a patient's needs. A good guideline to follow is possession of 6 mature plants or 12 immature plants, and/or 8 ounces of dried cannabis. If you will only possess these amounts for personal use, a state issued card can absolutely protect you from arrest.