If you've been following the "severe covid is serotonin syndrome" story for the past year like I have, the doctors behind the theory have a preprint up: https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3800402

This paper lays out the pathophysiology of severe covid and proposes cyproheptadine, an antihistamine, as a repurposed drug appropriate for treatment. It also sheds some light on why early treatment with some SSRI's like fluoxetine or fluvoxamine might prevent deterioration.

You may remember Dr Kyle-Sidell from his "COVID-19 is not typical ARDS" videos from a year ago where he explained that putting people on ventilators based just on low O2 saturation was needlessly harmful.

"Do COVID-19 Vent Protocols Need a Second Look?" https://www.medscape.com/viewarticle/928156

Dr Jalali had first proposed that covid-19 was a disease of hypercoagulation last May (maybe earlier) and has since worked out a theory that the cause of severe symptoms is driven by serotonin released from platelets by the virus.

Did you know that 95% of your body's serotonin is stored in your platelets? Almost nobody pays attention to this and it could be the key to curing severe covid.

Ok so I think what this is really referring to is from a later citation (68) about serotonin and homeostasis. Serotonin does not cross the blood-brain barrier. The central nervous system produces its own serotonin in the brainstem, but it is only 5% of total body serotonin.

The other 95% of serotonin in the body is produced in the gut. Most of that stays in the gut, and of what is released in the bloodstream, 98% of that is held by platelets. So only 2% of your body's serotonin circulates in plasma.
https://sci-hub.st/https://www.sciencedirect.com/science/article/pii/S001457931500455X

The overall aim of this paper is to explain peripheral serotonin physiology, show how platelets are activated to release serotonin in severe covid-19 and advocate for cyproheptadine (a generic antihistamine an serotonin antagonist) as a treatment for severe covid.

Cyproheptadine is currently considered controversial as a covid treatment because the serotonin physiology is not well understood, and I think because there's a general attitude of disdain towards repurposed drugs.

I've seen some of the authors of this paper get shouted down by other doctors for trying to discuss this, as if somebody was trying to push a conspiracy theory.

What's funny is that cyproheptadine for serotonin syndrome is an uncontroversial treatment, practically a home remedy, for cats with chronic kidney disease.
https://www.felinecrf.org/persuading_cat_to_eat.htm

It's not a rare thing for cats with ckd because they are frequently prescribed a drug combination for poor appetite and nausea (mirtazapine and ondansetron) that can be serotonergic together.