10/18/2026
🧠 Why do some people still struggle with memory problems and brain fog long after COVID? A study points to a surprising chain of events that starts in the gut and ends with low serotonin.
Serotonin is a chemical messenger best known for its role in mood. But it also helps control memory, sleep and digestion, and most of it is made in the gut.
Researchers at the University of Pennsylvania studied blood and stool samples from people with long COVID. They also ran experiments in mice.
In some patients, they found traces of the COVID virus in stool months after the infection. This suggests the virus can linger in the gut.
The leftover virus seemed to keep the immune system switched on, causing ongoing inflammation.
That inflammation made it harder for the gut to absorb tryptophan, a building block the body needs to make serotonin.
With less tryptophan, serotonin levels dropped. Patients with long COVID had lower serotonin in their blood than people who recovered fully.
Low serotonin then disrupted the vagus nerve, a major communication line between the gut and the brain. In mice, this was linked to memory problems.
When the researchers boosted serotonin in mice, using tryptophan building blocks or SSRI antidepressants, the memory problems went away.
The team says serotonin levels could one day help doctors identify which long COVID patients might benefit from certain treatments.
But there are important caveats. This pathway was found in only a subset of patients, and long COVID likely has many different causes.
The key cause and effect steps were shown in mice, not people.
Clinical trials are needed before anyone uses SSRIs or supplements to treat long COVID. People should not start these drugs without medical advice.
📃 RESEARCH PAPER
📌 Andrea C. Wong et al., "Serotonin reduction in post-acute sequelae of viral infection," Cell.