A physician recommended I try progesterone cream to combat MPB as opposed to Finasteride. What are the long-term side effects?

There is a long list of potential side effects. A lot would depend on the dosing and frequency of use of the medication. Does this physician specialize in hair restoration surgery? Is the prescription meant to slow down the progression of the hair loss or to get hair growth? 

The best treatment for hair loss in young men is the drug finasteride. You should see a doctor who specializes in hair loss and hair restoration