Why is it a sin to take drugs?
Using drugs is a sin because it is an act of self-destruction and thus an offense against the life that God has given us out of love. Every form of a person's dependence on legal substances (alcohol, medication, tobacco) and even more so on illegal drugs is an exchange of freedom for slavery; it damages the health and life of the person concerned and also does great harm to the people around him. Every time a person loses or forgets himself by becoming intoxicated, which can also include excessive eating and drinking, indulgence in sexual activity, or speeding with an automobile, he loses some of his human dignity and freedom and therefore sins against God. This should be distinguished from the reasonable, conscious, and moderate use of enjoyable things.
(Catholic Youth Catechism, question 389)