There are quite a few documents written by Dr.LaVey and others as to why Satanists should not use drugs. Personally I could careless about its effect on your health as that is your responsibility. You are correct, the Church of Satan
does not approve of any illegal activity and if it becomes an issue, yes you can be ex-communicated from the organization. What you do in the comfort of your own home is up to you, if you take it outside your home then it becomes a problem for more than just you and that is never good. Reponsibility to the Responsible is the term that fits here, though personally I find drugs to be a weakness and it shows bad character for those who use them.
But as I always say .... "To each their own until it effects me, then to each a Grave".