I think that it is more likely to slow down hair loss, because it lowers Testosterone levels.
''Effects of marijuana on testosterone in male subjects.
Barnett G, Chiang CW, Licko V.''
Abstract
''Clinical studies have given contradictory reports on the effect of smoking marijuana on the plasma levels of testosterone in males. A reanalysis of existing data established that testosterone levels are depressed both after smoking one marijuana cigarette and after intravenous infusion of delta-9-tetrahydrocannabinol, a pharmacologically active component of marijuana. Simulation of the marijuana interaction, under the assumption that delta-9-tetrahydrocannabinol inhibits testosterone production or secretion, suggests a minimum of 24 hours are required for testosterone to return to pre-smoking levels. A series of clinical studies are specified to clarify the nature of the interaction.''
I smoked daily for around four years, and my hair loss was pretty slow (while smoking) considering it started in my early teens. It really started accelerating after I quit....