Ketoconazole Shampoo
Ketoconazole shampoo (commonly 1% OTC or 2% prescription, often marketed as Nizoral) is an antifungal shampoo that many people also use off-label for hair loss / shedding because it can reduce scalp inflammation and dandruff-type irritation that sometimes accompanies thinning. Beyond its antifungal action, ketoconazole has anti-androgenic properties. In forums of those with PFS/PSSD/PAS communities, it is common to result in a crash, flaring or permanently worsening the syndrome. Evidence remains anecdotal as there are no clinical studies proving the shampoo can cause PFS.
Crash Anecdotes:
https://www.reddit.com/r/FinasterideSyndrome/comments/1eu1we5/has_anyone_crashed_on_ketoconazel/
My Personal Risk Ranking:
High Risk of Permanent Worsening (for PFS/PSSD/PAS):
If you currently have PFS/PSSD/PAS, this substance may carry a high risk of worsening symptoms—potentially in a lasting way—and is best avoided.
For those without these conditions, it is unlikely to trigger this condition on its own Given the uncertainty and the potential severity of outcomes, the risk may not be worth the advertised benefits.