Is it okay to smoke marijuana if I have a medical permit?

Question from a reader:

If everything God made is good, then what do you think about marijuana? It's obvious that God made opiates for a reason but man perverted their purpose and turned them into a sin. However, marijuana doesn't seem to have any purpose except to smoke it and relieve pain and anxiety (which is why I have a medical card). Obviously man turned this into a great evil too, but can it be used properly? I don't abuse it but I would be lying if I said I didn't enjoy getting high just like some people enjoy having a drink or two. What do you think? I don't feel particularly convicted about this either. Should I be?