Why does catch up hair loss occur after stopping propecia?

Prospects blocks DHT from locking onto predisposed hair follicles and staving off hair loss. As long as you use it, the effects last. Once you stop it, hair loss will occur. The rates of loss vary, but inevitable.

This is an excellent question. No one has formally studied the effects of discontinuation of Propecia. We know that once the inhibition of DHT is no longer present, that the hair will return to its genetic predisposition. It is my opinion that the speed of hair loss after the discontinuation, varies from Individual to individual. If you started with an aggressive type of hair loss, you may indeed jump Norwood patterns. If you have a gradual miniaturization, this may proceed slowly. Hope this helps. Good luck.

That's a great question.  I don't think the answer to your question has been answered.  Hair loss seems to be multi-factorial.  For this reason, some respond very well to Propecia, some don't respond at all, and in others Propecia simply reduces the rate of hair loss.  For hair to miniaturize in one cycle upon cessation of Propecia, one would assume that the DHT blockage interferes with the miniaturization by up regulating certain genes that in turn stimulate stem cells through the biosynthesis of their a variety of modulators.  Removal of DHT inhibition would in turn stop the up regulation so dermal papilla pre-destined to miniaturize immediately cease to produce coarser terminal hair follicles.  

The drug finasteride blocks the DHT at the hair level. When the drug is stopped, the block of the DHT which attacks the hair follicles, ceases, and the hair goes into a condition called apoptosis which is a death spiral for the hair follicles that are genetically impacted by the DHT.  As each hair cycle has a fixed life term, apoptosis kicks in once the drug is stopped and you get to back where you were prior to taking the drug in the first place. That is what is called 'Catch Up Hair Loss'. As long as you stay on the drug, the drug will block the impact of the DHT.