Took finasteride for 3 weeks then quit due to side effects, had significant shedding, will the hair regrow?

Some patients do shed when they start finasteride, however that hair generally does come back in time. I would recommend following up with your doctor if concerned.

It takes over 6 months of taking Propecia (finasteride) daily to see results.Taking for 3 week will have not much (if any) effect on your hair.If you lost more hair, it is your genetics winning out.Taking Propecia does not make your hair fall out.  If you lost your genetically susceptible hair, it will likely not grow back.Have you considered a twist to Observational Bias?  Once you start taking a medicine that is suppose to GROW hair, you become acutely aware and interested in your hair status.  While expecting impatiently for hairs to grow you notice hairs naturally falling out (as it would normally).  Since you would NEVER see results of Propecia in days to weeks, you are acutely aware of how much hair you are naturally losing.  This phenomenon would likely be worse for those with Obsessive Compulsive traits.The opposite to the above scenario are patients who swear they see results of Propecia working in 1 to 2 weeks.  This is also highly unlikely.  

Shedding is not uncommon after starting finasteride in the first few weeks and months.  Although the exact mechanism is not clear, it might have to do with the effect of DHT blocker on accelerating hair growth cycles to initiate a new growth phase.  You have to pass the first week of shedding you will be fine.Of course you have to discuss this with your primary doctor and be examined if still concerned.

In general terms, some individuals experience hair shedding during the first 1-2 months of taking finasteride (Propecia, etc). If one stops, hair shedding will eventually stop and some degree of improvement will occur over the next 9-12 months.Will hair regrow 100% do the same same density as it was before? Well, this is actually a very involved question. If one is in an active phase of genetic hair loss, with lots and lots of hair follicle miniaturization and lots of shedding to begin with, then the genetic hair loss can progress over the 9 months one is waiting for the hair to 'come back'. The result will be that the individual does NOT regrow hair to the same degree as 12 months prior (the genetic hair loss progressed). Also, if one sheds a lot of very small, thin hairs (miniaturized) hairs, then they make not all regrow.Be sure to speak to your prescriber about specific details. ANYtime ANYbody with androgenetic hair loss (be that male or female) experiences significant shedding, there remains a possibility that hair will NOT completely regrow.

one of the FDA approved medications for hair loss is propecia.  It mostly effects the crown area.  You may see some hair growth but it's mostly used for decreasing future hair loss.  However it does carry with it slot of side effects،if you are young and experiencing the side effect, you should stop it.  You may lose some hair initially but some Of them may come back.  I suggest starting on rogaine, vitamins and laser, they may help with your sudden shedding.