Serotonin is a neurotransmitter primarily produced in the gut.
While its functions are multifaceted, serotonin plays a large role in intestinal motility, or the movement of food throughout the digestive system. Serotonin increases when responding to gut irritants such as pathogens and toxins. This is essentially a stress response as the body attempts to eliminate them from the body.
Chronic intestinal inflammation, stress, and certain medications have been shown to increase serotonin to unhealthy levels. High levels are commonly associated with cancer, including gastrointestinal neuroendocrine tumors.
Serotonin is often labeled the “happy hormone,” but in reality its biology is far more complex. At high levels, serotonin can actually promote inflammation, cell proliferation, and, in some contexts, cancer growth.
Under normal conditions, only about 1% of dietary tryptophan—the amino acid used to make serotonin—is converted into serotonin.
But in advanced cancer or severe chronic inflammation, this conversion can rise dramatically. Some studies show that up to 60% of available tryptophan may be diverted into serotonin production, reflecting a stressed and dysregulated metabolic state.
Cancer cells can either produce serotonin themselves or import it from the circulation. Many tumors express high levels of serotonin receptors, which, when activated, drive pathways related to:
- Proliferation
- Migration
- Angiogenesis
- Resistance to apoptosis
Elevated levels of serotonin have been linked to:
- Gastrointestinal neuroendocrine tumors (carcinoid tumors)– (these are the most classic; many actively produce serotonin)
- Pancreatic cancer
- Colorectal cancer
- Gastric (stomach) cancer
- Hepatocellular carcinoma (liver cancer)
- Breast cancer
- Prostate cancer
- Lung cancer (especially small cell lung cancers, which can produce serotonin themselves)
- Ovarian cancer
- Gliomas / brain tumors
- Bladder cancer
How does serotonin promote cancer?
Serotonin has been shown to:
- Suppress oxidative metabolism, pushing cells toward glycolysis (known as the Warburg effect)
- Increase inflammation
- Promote fibrosis and tissue stiffness, creating a microenvironment favorable to tumor growth
- Interfere with normal cell cycle regulation and disrupt healthy differentiation
- Activate receptors that support angiogenesis, immune evasion, and metastasis
A 2019 Nature study showed that serotonin can attach to histones, the proteins that organize and regulate access to DNA. When serotonin binds to histones, it creates an epigenetic change—it alters how tightly certain genes are packaged, making some of them much easier for the cell to switch on.
This process isn’t limited to the brain. Serotonin made in the gut or released during inflammation can enter many different cell types, including cancer cells, and directly influence how they read and express their genes.
In cancer, this becomes problematic: serotonin can amplify the activity of genes that drive tumor growth, help cancer cells resist cell death, and support metastasis. In other words, serotonin doesn’t just act as a neurotransmitter—it can also reshape gene activity in ways that make cancer cells more aggressive.
This has led researchers to explore whether blocking serotonin transport or modulating serotonin receptors may help treat tumors. For some cancers, we will use substances that directly block serotonin.
A common way to reduce serotonin in patients is a substance called Cyproheptadine, which works by antagonizing multiple serotonin receptors (particularly 5-HT2 receptors), inhibiting serotonin-driven signaling pathways involved in cell proliferation, angiogenesis, and resistance to apoptosis. It’s just one therapy for multiple factors we have to address, but lowering serotonin is a major way to help get cancer patients healthy.
A few things to know:
- Serotonin stimulates cell division and connective tissue formation, promoting fibrosis and tumor-like growths.
- High serotonin impairs mitochondrial energy production by inhibiting oxidative metabolism, shifting cells away from efficient energy use.
- Serotonin increases inflammation, especially when combined with poor diet or stress, contributing to a tissue environment favorable to cancer.
- Elevated tryptophan (the precursor to serotonin), especially across a lifetime, correlates with increased risk of pituitary and other cancers.
- To mitigate serotonin’s harmful effects it’s important to: optimize thyroid function, reduce dietary tryptophan when in excess, and use nutrients like glycine which have “anti-serotonin” properties.
A few ways to lower chronically elevated serotonin:
- Reduce gut irritation
- Insoluble fiber: raw carrot salad and cooked mushrooms
- Activated charcoal occasionally (with guidance)
- Support healthy digestion + faster transit
- Warm meals, adequate carbs
- Gentle walking after eating
- Magnesium-rich foods
- Lower endotoxin (one of the biggest triggers of high serotonin)
- Avoid high-tryptophan, hard-to-digest proteins
- Use anti-serotonin foods + nutrients
- Vitamin B6 (helps shift tryptophan away from serotonin)
- Vitamin E (anti-inflammatory, modulates receptors)
- Magnesium (calms serotonin-driven excitatory pathways)
- Aspirin (lowers inflammatory serotonin signaling)
- Reduce inflammation
- Increase protective hormones
- Progesterone (opposes serotonin’s excitatory + proliferative effects)
- Pregnenolone (calms serotonin-driven stress circuits)
- DHEA (supports dopamine and reduces serotonin dominance)
- Lower stress hormones
- Eat within 30–60 minutes of waking
- Avoid intense fasting or over-exercising
- Consider anti-serotonin medications like cyproheptadine (clinically supervised)
Unfortunately, high serotonin in the body can be a cause for concern. Hormones are not “bad” or “good.” It’s about their production levels, their exposure to cells, and their ratios to other hormones.
It’s time to dispel the idea that the more serotonin, the better. We want healthy and normal levels, but certainly not too high.