Common (>10% users): Scalp irritation (itch, redness, mild flaking) — vehicle-dependent, often resolves with vehicle adjustment or short break.

Less common (1-10%):

• Contact dermatitis (more severe / persistent skin reaction)
• Initial shed (weeks 1-4) — hair-cycle artifact, not toxicity
• Mild fatigue, low energy — anecdotal
• Mild libido decrease — anecdotal, usually dose / area-related
• Headache — uncommon

Rare-serious (<1% but worth knowing):

• Gynecomastia / breast tenderness — anecdotal, suggests systemic AR antagonism is not zero in some users. Documented in the systemic-AR-antagonist class (flutamide, bicalutamide) and a known plausibility for RU58841 if absorption exceeds design.
• Mood disturbance, depressive symptoms — anecdotal, similar caveat. Class-effect possibility.
• Hepatotoxicity — flutamide and nilutamide have a clinically significant hepatotoxic signal at systemic doses; RU58841's design should preclude this at topical scalp doses, but no published human liver-enzyme dataset confirms it across long-term users.
• HPG axis perturbation — theoretical with high systemic exposure. Unstudied in chronic topical users.
• Unknown long-term effects of decades of daily topical AR blockade in young men — the longest anecdotal exposure pool is roughly 10-15 years and small. The pharmacological intuition is "should be safe given local-only design," but extrapolating from "should be safe" is exactly what got finasteride users into the post-finasteride-syndrome (PFS) territory in the first place. Humility is warranted.
• Quality-control failures from research-chem sourcing — the molecule itself isn't necessarily the problem, but vendor-by-vendor variation in purity, vehicle quality, contamination, and labeling is. A bad batch can cause irritation, allergic reactions, or under/over-dosing. This is the single most underappreciated risk category for research-chem hair-loss drugs.

Specific watch periods:

• First 4 weeks: Watch for scalp irritation, contact dermatitis, initial shed (don't panic-quit on shed alone).
• Months 1-3: Watch for systemic-feeling symptoms (fatigue, mood, libido, breast tenderness). Stop or reduce if any appear.
• Month 6: Photographic check-in for response; baseline LFTs and hormone panel if there's any concern about systemic exposure.
• Annually thereafter: Photo series, hormone panel if indicated.