Joachim Andersen

Accelerated Learning

Hormesis: How a Little Stress Makes Us Stronger

Hormesis illustrated by a silver-haired man in a black linen shirt with his eyes closed beside a steaming copper bath on a stone terrace at night, while a young woman with a dark ponytail holds a glass pipette over the bath and a single golden drop falls, next to a golden hourglass and a bowl of dark berries.
Hormesis illustrated by a carefully measured drop: the right dose of stress, given time, makes us stronger.


Hormesis is what happens when a small dose of something that is harmful in large amounts triggers a response that leaves me stronger than before. It’s one of the mental models I find most useful, because it explains why some stress is good for us, and why the line between medicine and poison is so often just the dose.

The essence of Hormesis is to ask: “What is the right dose of this stress, and am I giving myself enough time to recover and adapt?“


Origin: From Paracelsus to a Forgotten Curve

In 1538, the Swiss physician Paracelsus wrote that “All things are poison, and nothing is without poison. The dosage alone makes it so a thing is not a poison.” It’s usually shortened to “the dose makes the poison”, and it became the founding idea of toxicology.

In 1888, the German pharmacologist Hugo Schulz noticed something stranger. A dozen disinfectants that stopped yeast at high doses made it produce more carbon dioxide at very low doses. He first assumed he had made an error, but the result kept repeating. Together with the psychiatrist Rudolf Arndt, he turned it into the Arndt–Schulz rule. Because they used it to defend homeopathy, mainstream science largely ignored the idea for decades.

The name came in 1943, when Chester Southam and John Ehrlich found that weak extracts of western red cedar sped up the growth of wood-decaying fungi, while strong extracts stopped it. They called the effect hormesis, from the Greek for setting in motion. From the 1990s, the toxicologist Edward Calabrese and his colleague Linda Baldwin brought it back, by combing through thousands of studies and showing that the pattern appears across plants, microbes, animals and humans.


Key Concepts

  • The Biphasic Curve
    A low dose stimulates and a high dose inhibits, so the dose-response curve rises before it falls, like an inverted U. This challenges the common assumption that if a lot of something is bad, a little must be a little bad.

  • Modest and Narrow
    Calabrese found that the benefit is usually small, with a peak about 30–60% above normal and rarely more than double. The helpful zone is also narrow, typically within 10 – 20 times below the dose where harm begins.

  • Overcompensation
    A mild challenge disrupts the body’s balance, and it responds by building a little more defense and repair capacity than it needs. That extra capacity is what makes me stronger.

  • Stress Plus Recovery
    The adaptation happens after the stress, not during it. Without enough rest, the same dose that would make me stronger simply wears me down.

  • A Building Block of Antifragility
    In Antifragile (2012), Nassim Taleb uses hormesis as a key example of things that gain from disorder, which I touch on in Diversification.


Some Examples

  • Exercise and Antioxidants
    In a 2009 study led by Michael Ristow, young men who took high doses of vitamin C and E during four weeks of training lost much of the improvement in insulin sensitivity that the training gave the others. The small amounts of oxidative stress from exercise seem to be the signal that makes the body adapt.

  • Heat Shock Proteins
    In 1962, Ferruccio Ritossa saw new activity on the chromosomes of fruit flies after a colleague accidentally raised the temperature of an incubator. This was the first sign of the heat shock response, in which mild heat makes cells produce proteins that repair and protect other proteins. I look at the practical side of this in Heat and Cold Exposure.

  • Fasting
    Rafael de Cabo and Mark Mattson describe how intermittent fasting acts as a mild metabolic stress that switches the body from sugar to ketones and turns on cellular defenses. The evidence is strong in animals and still growing in humans.

  • Plant Compounds
    Mattson argues that many plant compounds are mild toxins that plants make to defend themselves, and that in small amounts they activate our own stress-response pathways. I go into specific foods in Natural Foods.


Applying Hormesis

When I want to use this mental model, this is the simple process I follow:

  1. Find the Right Dose
    Look for the range where a stress is challenging but not damaging, and remember that it differs from person to person.

  2. Start Small and Progress
    Begin with a low dose and increase it gradually as I adapt, rather than jumping straight to the extreme. Experiment and try to identify the sweet spot yielding the best results at the lowest dose: the minimum effective dose.

  3. Recover Fully
    Treat sleep and rest as part of the process, because that is when the adaptation happens.

  4. Don’t Blunt the Signal
    Be careful with habits that remove the stress right after it, like megadoses of antioxidants after training.

  5. Vary the Stressors
    Combine different mild stresses, physical, mental and social, rather than overloading a single one.

  6. Apply It to Learning
    Robert Bjork calls the useful struggles of learning “desirable difficulties”: testing myself and spacing out practice feels harder, but makes knowledge last longer.


Pitfalls and Limitations

  • Radiation hormesis is disputed. The US National Academies’ BEIR VII report (2006) concluded that the evidence supports a linear no-threshold model, with no safe dose for cancer risk, while a 2005 report from the French Academies argued that it overestimates the risk of very low doses. Radiation protection still assumes that less is better.

  • It’s not a license for toxins. The benefit is small and the window narrow, and just past it lies harm.

  • It can work against us. Calabrese notes that low doses of some anti-cancer drugs can stimulate tumor growth, and that a dose that helps one person can harm another.

  • Hormesis is not homeopathy. Homeopathic remedies are often said to be diluted until no molecules remain, which is a different and unsupported claim.

  • This is education, not medical advice, thus check with a doctor before embarking on fasting or other significant interventions.


Related Mental Models

Hormesis reminds me that the goal is not to avoid all stress, but to find the right dose and recover from it. By seeking small, regular challenges for my body and mind, and respecting the point where they turn harmful, I can grow stronger from the very things that would break me in larger doses. Personally I call this the “vaccine approach.”



To learn more about mental models and philosophy,
check out: Wise: How to Attain a Deep Understanding of the World, History, Our Place in It, and How to Live Well



📚 Sources and References


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