@whatevr is right.
If the Finasteride penetrates your scalp you will get the desired effect (no hairloss) but also sides. Or it never enters your scalp bc of poor penetration.
It is not possible to have topical finasteride to get to your hair bulbs but not into your bloodstream.
There are many threads here about Hassons and Wong topical finasteride that uses a special vehicle, but I think nothing really came of it.
What is somewhat effective and has less sides is ru58441. For example
@Swoop takes finasteride until he gets sides than switches to ru for half a year and then back.
I have slight sides on finasteride ( water sperm) and i try to low dose finasteride and use ru at the same time.
0.25-0.5 mg finasteride and 50mg ru per day.
As the illegal german pharmacy where I bought my fincar was raided, I use now one 5mg proscar pill dissolved in 10ml vodka for two weeks. ~1ml vodka per day.
That is still overkill /overdose but my hairloss is quite aggressive. I will likely dose ever lower in the future.
I would try to switch to ru if you can afford it and it's available. And experiment with low dose finasteride.
If your sides are only general loss of libido & agressiveness, lifting and sports helped for me. Our sedetary lifestyle is death for testo levels anyway even without finasteride.