"If I have a prescription from my doctor do I need a medical card?"
Depends on where you're buying. If you go to a storefront dispensary, often times they done let you in the door if you don't provide your letter of recommendation (some go so far as to require a raised seal on the document). So in most cases, a card is convenience but not a need. You just need to show your state ID and provide your letter of recommendation-most delivery services just need the info on the document for online verification and don't require the physical documents.