Those great artists who were or are known to be mentally ill or drug users make for interesting news stories and articles. That probably creates the impression that it's more common than it is. Also, the life style of successful artists some times encourages drug use. In some cases the artist might be great despite the use of drugs rather than with the help of drugs.

Jimrandomh makes a good point about not lumping all drugs together. That is, also, true of mental illness. Take bipolar disorder, for example. Van Gogh is thought to have had it. He probably produced large quantities of work during manic phases, but then he took his own life in a depressive phase. Those who suffer from severe depression or anxiety probably are not very productive.