Finasteride and Autoimmune Disease: What Clinicians and Patients Need to Know

What does the label say about autoimmune disease?

Propecia is a finasteride 1 mg tablet indicated for male pattern hair loss in men only, and it is not indicated for women [2][1]. Its contraindications section lists two items: pregnancy, and hypersensitivity to any component of the tablet [1]. The warnings and precautions section covers risk to a male fetus, effects on PSA, the high-grade prostate cancer signal seen at 5 mg, and pediatric use; neither section names an autoimmune condition [1].

The immune-related entry in the label is hypersensitivity. Rash, itching, hives and angioedema appear as postmarketing reports, and the Propecia label says these reports are voluntary, their frequency cannot be reliably estimated, and a causal relationship cannot be established [1].

What finasteride does hormonally

Finasteride is a competitive inhibitor of Type II 5-alpha-reductase, the enzyme that converts testosterone to dihydrotestosterone; the Type II isozyme drives about two-thirds of circulating DHT [1]. A 1 mg tablet suppresses serum DHT by 65% within 24 hours [1]. Mean testosterone and estradiol rise by about 15% but stay within the normal range, and finasteride has no affinity for the androgen receptor [1]. The label does not describe what these changes mean for someone with an autoimmune condition.

What the label says about interactions

The label states that no drug interactions of clinical importance have been identified and that finasteride does not appear to affect the cytochrome P450 system; antipyrine, digoxin, propranolol, theophylline and warfarin were tested with no clinically meaningful interactions [1]. The label adds that, although specific interaction studies were not performed, doses of 1 mg or more were used in clinical studies with drug classes including ACE inhibitors, analgesics, anticonvulsants, NSAIDs and quinolone anti-infectives without evidence of clinically significant adverse interactions [1]. Immunosuppressants are not among the classes it lists, and the interaction section does not discuss CYP3A4 inhibitors or inducers even though finasteride is metabolized mainly by CYP3A4 [1].