Finasteride is an antiandrogen. Effectively, it's chemical castration. To say that side effects are rare is an outright lie, since its primary action is to chemically castrate. In order for side effects to be effectively rare, subjects must already have low testosterone and/or low DHT. This way, subjects will report no significant change. Brilliant. Do this to a 20 yo male with testosterone in the 900s, he's gonna feel it, that's an absolute certainty.

It's possible to restore testosterone production in several ways. But leaving it alone isn't effective. It must be restarted actively. Otherwise, the estrogen will continue to inhibit a normal restart. This is where the "permanent" effect comes from, because it's said that testosterone production will return back to normal on its own, when in fact it's well-known that it does not, especially in the world of bodybuilding where they inject testosterone. These guys have developed all kinds of ways to avoid shut down of the testicles (more accurately, the HPTA axis) during a cycle, and are continuously looking for ways to restart testosterone production after a cycle, because they know.

Another drug used to chemically castrate is triptorelin, which incidentally was found to have the ability to restart testosterone production when taken in a single tiny dose, rather than a huge single dose for castration. This drug is also used to treat prostate cancer, just like finasteride, by chemical castration.

In the case of testosterone production in men, it's a bit special because of the way it normally works, and the way it can be shut down and then fail to restart.

Also, it doesn't help that testosterone production requires fat and cholesterol (testosterone is a sterol), when we eat a high-carb low-fat diet.

It's just off the top of my head and may not be completely accurate, but it's close enough to make the point.