I should add here that I personally am not so sure about the PG-causes-hair-growth theory, showing that I don't ALWAYS go along with what Dr. Proctor says! :wink: While it's true that many topical minoxidil studies have shown an inexplicable hairgrowth effect from even the placebo solution that was used (thus creating the idea that the PG was responsible), not ALL studies have shown such an effect that could be attributed to PG. A notable example of that was Vera Price's 1999 study, which found a "placebo effect" not only in the group getting the placebo solution containing propylene glycol, but ALSO in the control group that received no topical solution at all!
Also, I believe people mention the use of propylene glycol as a "preservative" or something that "increases shelf life" in topical minoxidil solutions simply because that's what Dr. Lee said once. But as far as I know, PG acts neither as an antioxidant nor as an antimicrobial, so I think that term is being misused. I think the real purpose of the PG is, as CCS said, to dissolve minoxidil to a greater extent than alcohol alone can, and to assist in its penetration through the skin. I believe Dr. Lee misspoke when he called it a "preservative", or whatever.