When the effects of androgens in the body are lessened,
e.g. through lowering DHT or by systemic hormone receptor
blockade, the body seeks equilibrium through a process
called upregulation. This can take the form of increased
hormone production and/or increased tissue sensitivity
to the remaining hormones. The reason side effects
usually gradually disappear with finasteride is probably
due to such upregulation. In a small percentage of
individuals, it may be that this process overshoots
the mark, resulting in significant hyperandrogenicity.
This is marked by such signs as greatly increased facial
oil, increased pimples, and greatly elevated libido.
It's possible that in certain cases such hyperandrogenicity
overcomes the hair-protective effect of, say, finasteride,
though this does not appear to be the case for the
vast majority of people.