As DHT is partially responsible for androgenetic scalp hair loss, dutasteride (a 5α-reductase inhibitor) at least has some evidence of slowing hair loss, as it blocks the action of the 5α-reductase enzymes that convert testosterone into DHT [1]. I am not a medical professional, so do your own research, but this is one of the few substance shown to have a positive effect on hair loss prevention. As this is usually a genetic problem your mileage may vary significantly.

[1] Bostwick DG, Cheng L (24 January 2014). Urologic Surgical Pathology. Elsevier Health Sciences. pp. 492–. ISBN 978-0-323-08619-6.