What causes PFS? The main theories

Still affected long after your last finasteride tablet, and want to know why? Nobody knows yet what causes post-finasteride syndrome (PFS), where finasteride’s side effects carry on after stopping.

Researchers are following three leads: the brain’s own steroids, genes switched off, and the androgen receptor, the part of a cell that picks up male hormones.

You may have been told the drug is long gone. It is. Half of each dose leaves your blood in about five to six hours, says the UK information for doctors.

For three years, Matt thought finasteride was harmless. Then the sexual side effects hit suddenly. Most of his other symptoms, he says, began in the three weeks after he stopped. Here’s why he blames the drug.

Play video: Matt on when his symptoms began (extract)

That timing is the puzzle. Why would things get worse as a drug leaves the body? “The mechanisms behind why this happens so severely in some people and not at all in others are still not fully understood,” Matt says later in the film.

Each of the three leads is an attempt to answer that puzzle.

Why do PFS symptoms carry on after stopping finasteride?

Nobody has found the reason yet. Every lead looks for something in the body that stays changed after the last tablet.

Sex is only part of PFS. A checklist doctors and researchers proposed in 2022 lists problems with concentration and memory, depression and suicidal thoughts. These can come with the sexual changes, or on their own. If you’re having suicidal thoughts, Samaritans are on 116 123 in the UK.

The 2022 checklist describes timing like Matt’s: a lasting sexual problem may only start once the drug is stopped, or get worse when it is.

A 2026 animal study from the University of Milan found something similar in rats. The rats changed little while on finasteride. A month after the last dose, they were restless and avoided anything new, signs the team reads as anxiety. Rats aren’t people, but the team says this fits what men with PFS describe.

The studies in men have two weak spots. The men were only tested after PFS began. As the checklist says, it’s “not clear whether these were present before treatment”. And every study that tested spinal fluid comes from one team in Milan.

Could PFS be caused by changes in brain steroids?

Possibly. Finasteride blocks 5-alpha reductase, an enzyme the body uses to make other chemicals. One is DHT, the hormone behind hair loss. The brain also uses it to make its own steroids, which help control mood and how nerves work.

Finasteride reaches the fluid around the brain and spine, the UK information for doctors says. In a 2017 study, the Milan team tested that fluid in men with PFS who had stopped. Some of these steroids were lower than in healthy men, others higher.

You may see one of them named online: allopregnanolone, which helps calm nerve cells. In that study it was normal in the spinal fluid and lower only in the blood, the team’s 2020 review says.

Dr Michael Irwig, a hormone specialist, described some of the first cases in 2011. In a 2012 film, he asked whether hormone changes in the brain could change nerve cells. Here he explains what finasteride blocks.

Play video: Dr Michael Irwig on what finasteride blocks (extract)

Could genes or the androgen receptor be involved?

They might be, though each idea rests on a few small studies.

Genes switched off. A gene can be switched off without being damaged, through chemical tags stuck onto DNA. Scientists call this epigenetics. In a 2019 study, the Milan team found these tags on the gene for the enzyme finasteride blocks. They turned up in the spinal fluid of about 6 in 10 men with PFS, against fewer than 1 in 10 healthy men, in groups of 16 and 13.

The authors couldn’t say whether the tags dated from before birth or came from finasteride. Our summary of that study has more.

The androgen receptor. Two small studies tested penis skin. In 2014, an Italian team found more androgen receptors in men with lasting problems after finasteride than in men who’d never taken it. In 2021, a Houston team found the receptor’s gene more active in men with PFS.

Three theories, and what they can't show yet

1

Brain steroids

Changed levels in the spinal fluid of men with PFS. Nobody knows if they were there before finasteride.

2

Genes switched off

Off-switch tags on one gene in the spinal fluid. Could date from before birth.

3

The androgen receptor

More receptors, or more receptor activity, in penis skin. Skin only, and no samples from before.

What can I do?

Finasteride helps many men keep their hair. If you have side effects, that’s a decision for you and your prescriber. If your mood changes, the UK leaflet says to stop taking it and contact your doctor as soon as possible.

You don’t need to know the cause to be taken seriously. Tell your GP your dates, for example “Finasteride 1 mg from January 2023, last tablet March 2024”, and everything that changed, including mood, concentration and memory. Our guide to which doctor to see may help.

You can report it without your GP agreeing.

Report it: it helps everyone

A doctor’s note helps you. A report to the medicines regulator helps everyone else: it is how patterns get noticed.

• No proof needed. A suspicion is enough, even years after stopping.
• Where: the Yellow Card scheme in the UK, MedWatch in the US.
• Use the right words. If symptoms continued after stopping, write Post 5-alpha-reductase inhibitor syndrome (MedDRA 10082430).

Our reporting guide walks you through it

What else do people ask?

Is PFS genetic? That isn’t known yet. A 2014 Italian study found some androgen receptor gene variants more often in men with PFS than in men without hair loss. Men with hair loss who’d never taken finasteride had them more often too, so they may simply go with hair loss.

Can a test show these changes? Not at your GP. The steroid and gene findings came from spinal fluid taken for research, and blood samples didn’t show the same. The 2022 checklist names no test for PFS. Our article on finasteride and testosterone explains why hormone tests can come back normal.

Could something else explain PFS? A 2021 Milan study found different gut bacteria in men with PFS. Some researchers doubt finasteride is behind PFS at all: a 2026 US study of men from a prostate cancer prevention trial found no sign of it. Others point to the nocebo effect, where expecting side effects brings them on. Our nocebo page weighs that argument.

Matt’s question, why some men and not others, is still open. If finasteride changed something for you that hasn’t changed back, report it with your dates, and read more about PFS.

Sources

1. Organon Pharma (UK). Propecia 1 mg film-coated tablets, package leaflet, electronic Medicines Compendium, last revised May 2026. Section 2: if you experience mood alterations, depression or suicidal thoughts, “stop taking Propecia and contact your doctor for further medical advice as soon as possible”. Section 4, frequency unknown: “persistent difficulty having an erection after discontinuation of treatment”, “persistent decrease in sex drive after discontinuation of treatment”, “persistent problems with ejaculation after discontinuation of treatment”.
2. Organon Pharma (UK). Propecia 1 mg, summary of product characteristics, text revised 29 June 2026. Section 5.2: “Mean terminal half-life is approximately 5-6 hours in men 18-60 years of age”; finasteride “has been recovered in the cerebrospinal fluid (CSF), but the drug does not appear to concentrate preferentially to the CSF”.
3. Healy D, Bahrick A, Bak M and colleagues. Diagnostic criteria for enduring sexual dysfunction after treatment with antidepressants, finasteride and isotretinoin. Int J Risk Saf Med 2022;33(1):65-76. Proposed criteria agreed by an expert group. Section 5 (PFS) lists “cognitive impairment”, “depression” and “suicidality”. These “can occur independently of any sexual difficulties but may also accompany sexual problems”. Altered neuroactive steroid levels, methylation of 5 alpha-reductase type 2 in cerebrospinal fluid and changed gut microbiota “have also been found in PFS patients”. Then: “But again, it is not clear whether these were present before treatment.” Section 9: “an enduring sexual dysfunction may only appear when the drug is stopped, or a mild dysfunction on treatment may worsen on stopping”.
4. Moral Medicine, This year I’m turning 30 years old, and I haven’t felt an orgasm since I was 24, interview with Matt, April 2026. Extract 11:18 to 11:37; the quotation is at 11:43 to 11:48 (automatic captions). His story on our site: Matt’s story.
5. Cioffi L, Diviccaro S, Chrostek G and colleagues, Melcangi RC. Finasteride withdrawal induces anxiety-like behavior and novelty avoidance in adult male rats. J Neuroendocrinol 2026;38(3):e70150. University of Milan. Rats treated for 20 days, studied again 1 month after stopping: effects during treatment “were mild”, “significant behavioral alterations emerged at the withdrawal”; results “in agreement with clinical reports in PFS”. Abstract read.
6. Melcangi RC, Santi D, Spezzano R and colleagues. Neuroactive steroid levels and psychiatric and andrological features in post-finasteride patients. J Steroid Biochem Mol Biol 2017;171:229-35. Sixteen men with PFS. In cerebrospinal fluid, decreased pregnenolone, progesterone, dihydroprogesterone, DHT and 17beta-estradiol, increased DHEA, testosterone and 3alpha-diol; plasma changes “did not reflect exactly what occurs in CSF”. Abstract read.
7. Diviccaro S, Melcangi RC, Giatti S. Post-finasteride syndrome: an emerging clinical problem. Neurobiol Stress 2020;12:100209. University of Milan; the authors received funding from the Post-Finasteride Foundation. Tetrahydroprogesterone is “also known as allopregnanolone”; it is among the steroids “able to modulate GABA-A receptors”. On the largest study (Melcangi 2017): “the levels of THP that were unaffected in CSF, showed a significant decreased in plasma”. Full text read.
8. PFS Foundation, Persistent sexual side effects of finasteride (Propecia) for male pattern hair loss, Dr Michael Irwig, August 2012. Extract 2:38 to 3:29; his question about hormone changes in the brain and nerve cells is at 3:37 to 3:51 (automatic captions). His study: Irwig MS, Kolukula S. Persistent sexual side effects of finasteride for male pattern hair loss. J Sex Med 2011;8(6):1747-53.
9. Melcangi RC, Casarini L, Marino M and colleagues. Altered methylation pattern of the SRD5A2 gene in the cerebrospinal fluid of post-finasteride patients: a pilot study. Endocr Connect 2019;8(8):1118-25. Sixteen men with PFS and 20 healthy men; cerebrospinal fluid from 13 of the healthy men. The SRD5A2 promoter was methylated in the spinal fluid of 56.3% of men with PFS and 7.7% of controls (9 of 16, 1 of 13). “No promoter methylation was detected in blood samples.” The authors “cannot conclude whether this pattern is prenatally established or induced by finasteride treatment”.
10. Di Loreto C, La Marra F, Mazzon G and colleagues. Immunohistochemical evaluation of androgen receptor and nerve structure density in human prepuce from patients with persistent sexual side effects after finasteride use for androgenetic alopecia. PLoS One 2014;9(6):e100237. Universities of Udine and Trieste. Eight men with sexual side effects lasting over 6 months after stopping, 11 men who never took finasteride. Density of androgen receptors in stromal and epithelial cells higher in cases. Abstract read.
11. Howell S, Song W, Pastuszak A, Khera M. Differential gene expression in post-finasteride syndrome patients. J Sex Med 2021;18(9):1479-90. Baylor College of Medicine, Houston, US. Penile skin cells from 26 men with PFS symptoms and 26 controls. “Androgen receptor expression was significantly higher in study patients”; no difference in the number of trinucleotide repeats. Limitations: “potential selection bias”, “lack of gene expression data prior to 5ARI exposure; lack of non-penile tissue samples”, “a lack of mechanistic data to imply causality”. Abstract read.
12. Borgo F, Macandog AD, Diviccaro S and colleagues, Melcangi RC. Alterations of gut microbiota composition in post-finasteride patients: a pilot study. J Endocrinol Invest 2021;44(6):1263-73. 23 men with PFS (21 analysed), 10 healthy men. Abstract read.
13. Cecchin E, De Mattia E, Mazzon G and colleagues. A pharmacogenetic survey of androgen receptor (CAG)n and (GGN)n polymorphisms in patients experiencing long term side effects after finasteride discontinuation. Int J Biol Markers 2014;29(4):e310-6. Extreme-length variants were more frequent in men with hair loss and PFS, and in men with hair loss who never took finasteride, than in men without hair loss. The authors read this as “probably reflecting the genetic predisposing factors for AGA development”. Abstract read. See also Cauci S and colleagues, Sex Med 2017;5(1):e61-71.
14. Till C, Tangen CM, Goodman P, Unger JM, Thompson IM Jr. No evidence of post-finasteride syndrome in 267,983 person-years follow-up in the Prostate Cancer Prevention Trial. Urology 2026 (online 22 August). Fred Hutchinson Cancer Center, Seattle, US. Men in a placebo-controlled trial of finasteride for prostate cancer prevention, followed through US Medicare claims: none of 22 conditions “potentially related to PFS” was linked to the finasteride arm; the authors conclude that follow-up “does not support the concept of a ‘Post-Finasteride Syndrome’”. Abstract read.

Sources checked on 28 October 2026. AI-assisted writing and review, not clinical review.