How can a natural erection drug that works?

Exercise. The best medicine for improving erections is increasing physical activity; this naturally boosts testosterone and blood flow to the penis. Supplements like yohimbine, l-arginine, maca, ginseng have all been studied and may have some benefit.
Unclear. If you are asking how to find a natural ed drug, I have never heard of a natural drug that works and is safe. Another expert may have a better answer for you. If instead ask how drugs help improve erections, most work by constricting the vessels that drain blood from the penis, thereby helping keep blood pooled in the organ.