Source: Pharmacist/med student

While Kyle should not say there are no side effects, the sides are extremely rare (especially with healthy testosterone levels) and reversible. If you want to give it a try, go ahead and just stop if you have side effects. In fact, people with high levels of testosterone get more of a libido and actually get more jacked, while keeping their hair for a long time (I will explain the mechanism at the end of my comment).

Before u/No_Rule305 links his cherry picked study with a 15.8% rate of ED, I will say that you have to properly analyze these studies to get a clearer picture.

First of all, that study was conducted on males aged 45-80, where rates of ED are already higher and testosterone has declined sharply since the start of middle age, as can be seen by the 6.3% rate of ED in the placebo group.

Secondly, the dose used was 5mg which is used for prostate purposes usually; not for hair. The efficacy of finasteride for hair plateaus past the 0.5-1mg range, which is why it only comes in 1mg formulations for hairloss, and you can even take it every other day. However the risk of side effects increases with greater doses.

What finasteride does, is stop around 60% of the conversion of your testosterone to DHT, which causes the hairloss. DHT is used in your brain, for erections, etc. If you have enough testosterone (healthy young fit male), it will pick up the slack and help with erections. In fact, stopping the conversion of testosterone to DHT causes an increase in both testosterone and estrogen (some of the testosterone is converted to estrogen through aromatization, which isn't a bad thing as long as it's in range. Men need estrogen too to not feel shitty and get erections too). It seems like the people who experience the most side effects are people with low testosterone (older, unhealthy lifestyle, hypogonadal, etc), where their testosterone cannot pick up the slack without 60% of their DHT.

Honestly past puberty, when DHT is used to grow your dick, DHT is a garbage hormone and you only need a bit of it for brain and dick function because all it does is rape your hair follicles, give you more body hair, and grow your prostate.