Dear Sarah:

It is a sin to use any illegal substance. There are some states that permit medical marijuana. It is only in those states that its medical use can be morally licit.

However, the Christian ethic involves more than whether or not something is legal. There are huge numbers of people who use the medical marijuana laws as an excuse to smoke marijuana with impunity. Even in states where medical marijuana is legal, if the person's intent is to use those laws as an excuse it will be morally illicit and thus a sin.

Unfortunately, medical marijuana is not properly regulated like other pain drugs, like oxycodone and morphine. Thus, doctors may be a little to "free" about recommending its use. With other pain medications doctors with integrity take care to be sure it is really needed and that the person is not abusing the drug. With medical marijuana the doctor recommends it and then it is all but regulated. The patient can use it for legitimate pain relief or for recreation with impunity.

Therefore, even in states where medical marijuana is legal, a person of good will and faith, must examine themselves and their intention. It's use must truly be for medical reasons whereby no other pain drugs will work.

As for the recreational use of marijuana, or other pain meds, it is decidedly illicit and sinful. Marijuana cannot be used for social reasons like alcohol. Marijuana always intoxicates. Alcoholic beverages only intoxicate when consumed in quantity. Thus, alcohol can be used in social intercourse. Marijuana cannot be so used. 

The point here is that intoxication, unless it is in conjunction of legitimately medical use, is sinful according to the Bible and the Church. Here are quotes from the Catechism:

1852 There are a great many kinds of sins. Scripture provides several lists of them. The Letter to the Galatians contrasts the works of the flesh with the fruit of the Spirit: "Now the works of the flesh are plain: fornication, impurity, licentiousness, idolatry, sorcery, enmity, strife, jealousy, anger, selfishness, dissension, factions, envy, drunkenness, carousing, and the like. I warn you, as I warned you before, that those who do such things shall not inherit the Kingdom of God." (Gal 5:19-21; cf. Rom 1:28-32; 1 Cor 9-10; Eph 5:3-5; Col 3:5-8; 1 Tim 9-10; 2 Tim 2-5.)

2290 The virtue of temperance disposes us to avoid every kind of excess: the abuse of food, alcohol, tobacco, or medicine. Those incur grave guilt who, by drunkenness or a love of speed, endanger their own and others' safety on the road, at sea, or in the air.

2291 The use of drugs inflicts very grave damage on human health and life. Their use, except on strictly therapeutic grounds, is a grave offense. Clandestine production of and trafficking in drugs are scandalous practices. They constitute direct co-operation in evil, since they encourage people to practices gravely contrary to the moral law.

I cannot speak to the issue of whether or not marijuana is an approprite medicine. That is for the scientist to decide.

If one uses medical marijuana recommended by a doctor and in a state that legally allows medical marijuana, and the person uses it properly and with right intent, then it may be morally permitted.

God Bless,
Bro. Ignatius Mary