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5α-Reductase inhibitors like finasteride and dutasteride can slightly increase circulating levels of testosterone by inhibiting its metabolism .  However, these drugs do this via prevention of the conversion of testosterone into its more potent metabolite dihydrotestosterone (DHT), and this results in dramatically reduced circulating levels of DHT (which circulates at much lower relative concentrations).   In addition, local levels of DHT in so-called androgenic (5α-reductase-expressing) tissues are also markedly reduced,   and this can have a strong impact on certain effects of testosterone.   For instance, growth of body and facial hair and penile growth induced by testosterone may be inhibited by 5α-reductase inhibitors, and this could be considered undesirable in the context of, for instance, puberty induction .   On the other hand, 5α-reductase inhibitors may prevent or reduce adverse androgenic side effects of testosterone like scalp hair loss , oily skin , acne , and seborrhea .  In addition to the prevention of testosterone conversion into DHT, 5α-reductase inhibitors also prevent the formation of neurosteroids like 3α-androstanediol from testosterone, and this may have neuropsychiatric consequences in some men. 
Fluid and electrolyte disturbances: Retention of sodium, chloride, water, potassium, calcium, and inorganic phosphates.
Gastrointestinal: Nausea, cholestatic jaundice, alterations in liver function tests, rarely hepatocellular neoplasms and peliosis hepatis (see WARNINGS ).
Hematologic: Suppression of clotting factors II, V, VII, and X, bleeding in patients on concomitant anticoagulant therapy, and polycythemia.
Nervous system: Increased or decreased libido, headache, anxiety, depression, and generalized paresthesia.
Allergic: Hypersensitivity, including skin manifestations and anaphylactoid reactions.
Vascular Disorders: venous thromboembolism
Miscellaneous: Inflammation and pain at the site of intramuscular injection.