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In 1989 a Pfizer team led by Simon Campbell in Sandwich, Kent synthesized compound UK92480, hoping to treat high blood pressure and angina. Trials at Morriston Hospital in Wales showed almost no effect on chest pain, but researchers led by Ian Osterloh noticed men reporting erections. Pfizer repositioned the drug, patented it in 1996 and won FDA approval in 1998, the same year three scientists won the Nobel Prize for discovering nitric oxide signaling. This episode explains how sildenafil blocks the PDE5 enzyme so cGMP can keep blood vessels relaxed, and why it does nothing without arousal.

The same molecule went on to treat pulmonary arterial hypertension under the name Revatio and is used off label for Raynaud's phenomenon. The hosts cover its side effects, the lethal drop in blood pressure when combined with nitrates or poppers, and the advertising push with Bob Dole and Pelé that drove sales to nearly 2 billion dollars in 2008. Then comes the shadow market: counterfeit pills, recreational use, athletes, and a 2026 recall of a chocolate syrup secretly laced with sildenafil.

• Blue tinted vision, or cyanopsia, happens because sildenafil also binds PDE6 in the retina
• Patients sometimes stacked several lower dose generic Revatio pills to match an erectile dysfunction dose
• Pfizer's main US patent expired in 2020 after legal fights with Teva, and the UK began selling Viagra Connect over the counter
• A 2023 Interpol report found illicit erectile dysfunction drugs made up 22 percent of pharmaceutical seizures
• Fakes have contained blue printer ink, amphetamines, boric acid and rat poison

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In Greek myth an eagle ate Prometheus's liver every day and it grew back every night. The ancient Greeks got the biology right: the liver is the only internal organ that genuinely regenerates, and it can regrow from as little as 25 percent of its mass. This episode tours the roughly 3.3 pound organ that performs about 500 vital functions, from its surgical dividing line, Cantlie's line, to the fact that 75 percent of its blood arrives from the gut through the hepatic portal vein. Inside, hexagonal lobules mix that nutrient and toxin rich blood with oxygenated arterial blood so hepatocytes can sort, store and neutralize what we eat.

The hosts walk through glycogen storage, bile breaking fat into droplets, ammonia turned into urea, and toxication, where breaking down a drug such as paracetamol in excess creates a more harmful metabolite. They explain what happens when a living donor gives away up to 60 percent of the liver, and why failure brings jaundice, pale stools, dark urine and abdominal fluid. The episode ends with the liver in culture, from Plato and the Talmud to hepatomancy and a Sudanese drink made from giraffe liver.

• The liver keeps a one to two year supply of vitamin A and a three to five year supply of vitamin B12
• It holds about 10 percent of the body's blood and can store an extra liter in heart failure
• Non-alcoholic fatty liver disease is estimated to affect a third of the world's population
• The sources link regular coffee drinking to protection against cirrhosis, fibrosis and liver cancer
• Polar bear liver is toxic to humans because it stores extreme levels of vitamin A, and Tungusic peoples made arrow poison from rotting livers

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Imagine blowing a dangerously high number on a breathalyzer without having had a drink in weeks. This episode explains auto-brewery syndrome, also called gut fermentation syndrome, in which yeasts such as Saccharomyces cerevisiae, the same species used by bakers and brewers, along with strains of Candida and some bacteria, colonize the gut and ferment carbohydrates into ethanol. Normally the liver's alcohol dehydrogenase clears the trace alcohol digestion produces. In this condition production overwhelms it, and patients have been recorded with blood alcohol levels of 200 to 500 milligrams per deciliter.

Heavy antibiotic use can wipe out the gut's bacteria while leaving yeast untouched, and conditions such as type 2 diabetes, liver cirrhosis, Crohn's disease and short bowel syndrome raise the risk. Patients face disbelief from spouses and doctors until a locked down 24 hour carbohydrate challenge proves the alcohol is homemade. The episode also covers a bladder version seen in uncontrolled diabetes, the condition's use as a drunk driving defense, and treatment with very low carbohydrate diets, antifungals such as fluconazole and nystatin, probiotics and fecal microbiota transplants.

• French physician Charles Bouchard wrote about intestinal fermentation and auto-intoxication in 1887
• A 1948 case in Uganda: a five year old boy whose stomach ruptured after yeast fermented the sweet potatoes he had eaten
• In urinary auto-brewery syndrome the patient stays sober while the urine itself turns alcoholic, in one report continuing to ferment in the collection cup
• A 25 year old man in 2019 who blew 0.3 with no drinks recovered on three weeks of daily fluconazole
• Mark Mongeardo documented his fight with the condition on TikTok in 2025

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In 1976 researcher Harry McGurk and his assistant John MacDonald were running an infant study on how babies perceive language when a technician dubbed a video of a face saying one syllable over audio of another. When they played it back, they heard neither sound. Their brains produced a third. This episode explains the McGurk effect: hear ba ba while watching lips form ga ga and you perceive da da, a fusion the brain builds when the eyes and ears disagree. Researchers who have studied it for more than 20 years still fall for it every time.

The episode then follows the experiments that tried to break it. The vowel ee makes the illusion strongest while oo nearly destroys it, a female voice over a male face still fuses, and you can even trick yourself in a mirror. Brain damage, dyslexia, Alzheimer's disease, schizophrenia and autism spectrum disorder all change how strongly people experience it, and Japanese and Chinese listeners show a much weaker effect than speakers of English, Dutch, Spanish or German. The hosts close on what it means for eyewitness testimony and for lip synced deepfakes.

• Why the brain compromises on da: ba is made with closed lips, ga at the back of the throat, and da sits between them
• Audio can lag the video by up to 180 milliseconds and still fuse, but arriving 60 milliseconds early breaks it, just as light outruns thunder
• The speaker's mouth must fall within 60 degrees of your gaze, and the effect is stronger when you see the right side of their mouth
• A tactile task, like identifying a hidden object by touch, greatly reduces the illusion
• Babies show the effect at four to five months, and English speaking children grow more visually influenced after age six while Japanese children do not

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For decades the medical establishment treated the adult brain as fixed hardware. This episode traces the evidence that it is not, starting with Italian anatomist Michele Vincenzo Malacarne, who in 1793 trained one animal of a littermate pair for years and found its cerebellum substantially larger. Santiago Ramón y Cajal coined the term neuronal plasticity in the early 1900s over objections from peers such as Walter Spielmeyer. Eleanor Maguire's MRI scans of London taxi drivers, who memorize the city for the exam known as the Knowledge, found larger posterior hippocampi that tracked their years on the job.

The episode separates structural plasticity, such as new dendritic spines and neurogenesis, from functional plasticity. David Hubel and Torsten Wiesel sutured one eye of kittens shut and saw the visual cortex rewire to the open eye, Michael Merzenich watched neighboring finger maps annex the territory of a severed nerve in monkeys, and Paul Bach-y-Rita's vibrating chair let blind people perceive images through their backs. It then turns to maladaptive plasticity in addiction, phantom limb pain and chronic pain, and to treatments from Lorimer Moseley and Peter Brugger's imagined movement exercises to the rapid antidepressant effects reported for ketamine.

• The empiricist versus nativist debate and the idea of constrained plasticity
• How repeated drug use reshapes the nucleus accumbens, likened to a sled carving an icy rut
• Phantom limb sensations in 60 to 80 percent of amputees as neighboring maps invade the hand region
• Chronic pain linked to reduced gray matter in the prefrontal cortex
• A 2009 study of brain changes after 15 months of musical training, and nicotine's effect on acetylcholine receptors

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Each eye has a hole in its vision, and the brain hides it. This episode explains the physiological blind spot, the punctum caecum, which sits where the optic nerve exits through the optic disc, an area with no photoreceptors at all. It lies about 12 to 15 degrees toward the temple and 1.5 degrees below the line of sight, measures about 7.5 by 5.5 degrees, roughly the size of a fist at arm's length, and is joined by angioscotomas, the shadows cast by retinal blood vessels. The visual cortex covers the gap using the other eye's view and, with one eye closed, by interpolating patterns such as brick walls from the surrounding area.

Octopus and squid eyes evolved separately and route their nerve fibers behind the retina, so they have no blind spot, while the vertebrate layout traces back to the retina forming as an outgrowth of the brain. Anatomists long assumed the optic nerve's entry point was the most sensitive part of the retina until French scientist Edme Mariotte showed around 1668 that it detects no light, demonstrating it to King Louis XIV and the Royal Society of London. The episode ends with the central night blind spot, caused by a fovea packed with cones and nearly empty of rods.

• The R and L test that makes a letter vanish when viewed with one eye
• Why a jagged black void would distract more than an invented patch of background
• Convergent evolution of camera-style eyes in vertebrates and cephalopods
• Dark-adapted rods becoming an estimated 10,000 times more sensitive than cones
• Averted vision, looking 5 to 10 degrees off center to catch faint lights, in aviation and astronomy

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Pressing your palms against closed eyes produces a light show without a single photon entering the eye. These lights are phosphenes, from the Greek for to show light. This episode explains pressure phosphenes, including the shifting grid one scientist compared to a crumpling, fly-spotted fly screen, and how squeezing retinal ganglion cells makes them fire signals the brain can only read as light. It covers Isaac Newton sliding a blunt bodkin behind his own eyeball, 1800s French physicians pressing on blind eyes before cataract surgery to confirm the retina still worked, the prisoner's cinema reported in total darkness, and the heart drug ivabradine, which can cause phosphenes as a side effect.

In optic neuritis, damaged nerve insulation lets a loud sound leak into the optic nerve and appear as a flash of light. Researchers in the late 1980s proposed that abstract Paleolithic cave art records phosphene form constants. In 1968 electrodes placed on a blind woman's visual cortex produced points and bars of light, and early 1970s work used such grids to spell Braille. Later turns include astronauts seeing flashes from cosmic radiation, radiotherapy patients seeing blue Cherenkov light inside the eye, and the 2019 BrainNet experiment in which three people in separate rooms played a Tetris-style game linked brain to brain.

• Why stimulating tissue above the calcarine fissure produces light in the lower half of vision
• Transcranial magnetic stimulation and the magnetophosphenes it creates
• The debate over whether tACS stimulates the brain or leaks current to the retina
• Camera-fed bionic eye implants that turn the world into a grid of phosphenes
• BrainNet's 15 and 17 hertz lights, EEG caps, a TMS helmet and an 81 percent success rate

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Déjà vu, French for already seen, strikes roughly two thirds of people at least once. French philosopher Émile Boirac coined the term in 1876 in The Future of the Psychic Sciences. This episode explains who experiences it most, including younger people, frequent travelers, heavy film watchers and people who remember their dreams, and why a 2008 study found no diagnostic link to schizophrenia. It does accompany temporal lobe epilepsy, often as a seizure aura, with links to variants of the LGI1 gene on chromosome 10, and documented cases from 2001 and 2007 show drug combinations that raised dopamine or serotonin triggering repeated episodes.

Robert Efron at the Boston Veterans Hospital proposed in 1965 that a tiny delay between signals arriving from the two hemispheres makes the brain register one moment twice. Other theories include split perception and cryptomnesia, tested in a 2010 hypnosis study by O'Connor, Moulin and Conway using the puzzle game Railroad Rush Hour, and a 2012 virtual reality study showing that familiar room layouts can trigger the feeling. The episode covers Carl Jung's collective unconscious, jamais vu, presque vu and déjà vécu, and a 2018 fMRI study showing that frontal regions, not memory centers, light up during the experience.

• The computer mouse double-click analogy for Efron's dual processing theory
• Hypnotized volunteers told to feel familiar with a puzzle they had never played
• Chris Moulin's study in which 68 percent of 95 volunteers writing door 30 times developed jamais vu
• Déjà vécu, in which patients lose the ability to tell false familiarity from reality
• The word list built around window, used to induce déjà vu inside an fMRI scanner

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In the early 1970s, researchers hunting for a painkiller without morphine's addiction risk asked why the brain had receptors that fit a poppy extract so precisely. In 1973 they reasoned the body must make its own matching molecule, and they isolated natural opioid peptides from pig brains, called enkephalins, then similar peptides from calf brains. The word endorphin, joining endogenous and morphine, followed. This episode explains how the pituitary gland makes endorphins by cutting a large precursor protein, pro-opiomelanocortin or POMC, with the enzymes PC1 and PC2, and how in 1976 scientists isolated alpha, beta and gamma endorphins from camel pituitary glands.

A 1977 study found beta-endorphin more potent than morphine, and naloxone blocked its effects. Alpha and gamma, shorter pieces of the same sequence, fold differently and do not bind opioid receptors at all, acting instead like a stimulant and a neuroleptic. In the body, beta-endorphin stops peripheral nerves from releasing substance P, and in the brain it suppresses GABA, releasing a flood of dopamine. Then the episode turns to the runner's high: runners given a drug that blocked their opioid receptors still felt the high, and the evidence now points to endocannabinoids, which cross the blood-brain barrier far more easily.

• Why endorphin is named in part for Morpheus, the Greek god of dreams
• The nightclub bouncer analogy for how blocking GABA lets dopamine flow
• How the dual system may have let an injured early human keep running from a predator
• Triggers including inflamed tissue, electroacupuncture, deep meditation and hard laughter
• Endorphins as the pain shield during exercise while endocannabinoids deliver the euphoria

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Freezing to death is rarely the quiet drift into sleep shown in films. This episode follows the body as its core temperature falls below the normal range of 36.5 to 37.5 degrees Celsius. Below 35 degrees, mild hypothermia brings violent shivering, a racing heart and cold-induced diuresis, as blood pulled from the limbs makes the body sense too much fluid in its core. Below 32 degrees shivering stops because cold muscles can no longer burn their fuel, and amnesia and slurred speech set in. The episode also explains why alcohol, despite the image of the brandy-carrying St. Bernard, opens blood vessels in the skin and speeds the loss of core heat.

Below 28 degrees come paradoxical undressing, seen in 20 to 50 percent of hypothermia deaths, and terminal burrowing, also called hide-and-die syndrome. Water draws heat away about 25 times faster than air, and many cold-water deaths come from cold shock and cold incapacitation within minutes. The episode covers the rule that no one is dead until warm and dead, a Polish toddler named Adam who survived a core temperature of 12.7 degrees, the cold that ravaged the armies of Hannibal and Napoleon, and how surgeons now cool patients to as low as 10 degrees and stop the heart during deep hypothermic circulatory arrest.

• Two theories of paradoxical undressing: a misfiring hypothalamus and exhausted blood vessel muscles
• Why undressed victims found outdoors are sometimes mistaken for assault victims
• The Titanic's water at minus two degrees Celsius and the involuntary gasp reflex
• The Coast Guard's 50-50-50 rule on life jackets in 50 degree Fahrenheit water
• Rewarming with ECMO, and tardigrades that use the sugar trehalose to survive freezing