Finasteride is a medication used to treat androgenetic alopecia (male pattern baldness) and enlarged prostates. Reported side effects include male breast enlargement, lowered sex drive, and headaches (among others). Additionally, there have been many anecdotal reports of "brain fog" and some reports of individuals unable to recall short-term memories.
Finasteride blocks the conversion from testosterone to dihydrotestosterone (DHT) by inhibiting 5α-Reductase type II and III. However, it can also have an effect on neurosteroids. From Wikipedia:
Neurosteroids like 3α-androstanediol (derived from DHT) and allopregnanolone (derived from progesterone) activate the GABA_A receptor in the brain; because finasteride prevents the formation of neurosteroids, it functions as a neurosteroidogenesis inhibitor and may contribute to a reduction of GABA_A activity. Reduction of GABA_A receptor activation by these neurosteroids has been implicated in depression, anxiety, and sexual dysfunction.
Additionally to these secondary effects resulting from lowered DHT, finasteride itself also crosses the blood–brain barrier.
Beyond the reduction in GABA_A receptor activation, what effects does finasteride have on the brain (secondary via 5α-Reductase inhibition and lowered DHT, or directly, via crossing the blood-brain barrier) and what kind of functions are altered by it? In particular, is there any likely explanation for headaches, "brain fog," and difficulty with short-term memory recall that some users report?
Perhaps some results of this mouse model generalizes to human brains:
Finasteride treatment and its withdrawal induced neuroinflammation in hippocampus.
Although, the hippocampus is mostly responsible for long-term, rather than short-term, memory.