Serotonin is almost universally portrayed in the media and by conventional psychiatry as the “happy hormone” — a molecule that lifts mood and is worth boosting with SSRIs (Prozac, Zoloft, etc.). Ray Peat and I have pointed out for many years that this is a dangerous oversimplification. In reality, 90% of serotonin is produced outside the brain (mostly in the gut), and it acts as a stress hormone, promoting vasoconstriction, platelet aggregation, inflammation, and most importantly — fibrosis. Elevated serotonin has long been known to cause heart valve disease in carcinoid syndrome and in patients taking fenfluramine-phentermine (“Fen-Phen”). The study below now confirms that SSRIs (serotonin reuptake inhibitors) accelerate degenerative mitral regurgitation by increasing serotonin availability.
As the study below demonstrates, researchers analyzed over 9,000 patients who had surgery for degenerative mitral regurgitation (DMR) and found a clear pattern: patients taking SSRIs needed surgery at a significantly younger age than those not on these drugs. The mechanism is straightforward: SSRIs block the serotonin transporter (SERT), leaving more serotonin in circulation and in tissues. In the mitral valve, this excess serotonin stimulates valve cells to produce excess collagen, leading to thickening, stiffening, and eventual failure.
The study also identified a genetic variant (5-HTTLPR “long-long” variant) that reduces SERT activity — essentially mimicking the effect of SSRIs. Patients with this variant were more likely to require surgery, and their valve cells were hyper-responsive to serotonin. Animal models confirmed the causal link: mice lacking the SERT gene developed thickened mitral valves, and normal mice given high-dose SSRIs showed similar changes.
This completely contradicts the mainstream narrative that serotonin is a benign “mood regulator.” Serotonin is a potent pro-fibrotic agent. The heart valve findings are not isolated: follow-up studies showed that low SERT activity also contributes to aortic stenosis (calcification and stiffening of the aortic valve) and myocardial fibrosis. Blocking the serotonin receptor HTR2B reduced these harmful changes, pointing toward a potential therapeutic target.
The human-equivalent dose is not applicable here as this was an observational human study and animal dosing study (SSRI doses in mice are not directly translatable to human depression treatment doses). However, the takeaway is clear: long-term SSRI use accelerates existing valve disease. The researchers downplay the risk by stating SSRIs are “generally safe for most patients,” but they admit that once a valve has started to degenerate, serotonin and low SERT activity accelerate progression. Given that millions of people take SSRIs for years or decades, this represents a significant, underappreciated public health risk.
https://doi.org/10.1126/scitranslmed.adc9606
“…A large multicenter study has found that this common brain chemical could contribute to faster worsening of degenerative mitral regurgitation, a widespread and potentially serious heart condition.”
“…The research team analyzed records from more than 9,000 patients who had surgery for DMR… They found a clear pattern: patients taking SSRIs tended to need surgery at a younger age than those who were not on these medications.”
“…Mice engineered without the SERT gene developed thicker mitral valves. Normal mice given high doses of SSRIs showed similar changes, suggesting that reduced serotonin reuptake may directly affect valve structure.”
“…People with two copies of the so-called ‘long’ variant have lower SERT activity… Among patients with DMR, those with this ‘long-long’ pattern were more likely to require surgery… At the cellular level, this variant made valve cells more responsive to serotonin, leading them to produce extra collagen. This buildup can stiffen and distort the valve over time.”
“…Mice lacking the serotonin transporter developed not only thickened mitral valves but also myocardial fibrosis, reduced cardiac function, and age-related remodeling changes… The work also found that mitral valve cells were especially sensitive to serotonin-driven, pro-fibrotic signaling.”
“…In aortic stenosis, when SERT activity was low and HTR2B signaling was higher, the aortic valve showed more early scarring and calcium buildup. In mice, blocking HTR2B reduced many of these harmful changes, suggesting it could be a possible target for future treatments.”
