Overview
Penicillin is one of the old, safe antibiotics. Despite it hardly being used by doctors anymore, it is still very effective for the things it was originally used for. The newer antibiotics can do things like rupture tendons and damage connective tissue; the older ones, penicillin, erythromycin, and tetracycline, simply do not have those risks. The most useful effects are often anti-inflammatory rather than purely germicidal: clearing endotoxin-producing bacteria out of the intestine lowers cortisol and estrogen, raises progesterone, and resolves a wide range of symptoms from headaches and PMS to mood problems and infertility. The standard two-week, full-strength prescription is usually much more than is needed; a fraction of that dose, watched against actual symptoms over a day or two, is generally all it takes.
Key Points
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Penicillin is one of the old, safe antibiotics that has not been outdone by newer drugs. Erythromycin, penicillin, and tetracycline have decades of safety data and remain very effective. Recent antibiotics are able to do things like cause tendons to break and damage connective tissue. The modernized variants close to the originals (anthocillin, doxycycline, minocycline) are still pretty safe, but the original old molecules are the safest place to start.
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The benefit is often anti-inflammatory and intestine-cleansing rather than just germicidal. Reducing bacterial overgrowth and endotoxin in the gut lowers cortisol and estrogen and raises progesterone, because the intestine is a chronic drain and source of stress. In both rats and people, anything that cleans up the intestine produces the same hormonal shift, whether it is a series of penicillin doses or a daily raw carrot salad.
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A fertility clinic study found that moderate-dose penicillin relieved women's headaches and mood problems, shifted their hormones, and produced pregnancies. The doctor's original theory was that the ovaries might be infected. Before treatment the women had high cortisol and a high estrogen-to-progesterone ratio. After the antibiotic, cortisol and estrogen decreased and progesterone increased.
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The standard two-week prescription is usually overkill. Taking a 250 milligram dose, then another every four hours later usually produces sudden subsiding of symptoms after two or three doses. After three or four doses the symptoms are often gone. Tapering to eight-hour intervals (and halving the dose) for the next day or so makes sure they do not come back.
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For bowel infections and colds, often one-third of the standard dose is enough. What presents as a respiratory cold is most often an intestinal viral or bacterial irritation producing symptoms in the upper respiratory system. Half a penicillin tablet four or five hours apart can resolve symptoms in eight or ten hours. The pediatric rationale of treating a "secondary infection" actually works in practice because the antibiotic is treating the gut, which is the real source of the inflammation.
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The textbook two-week course exists to handle spore germination, not therapeutic need. A few studies found that infections could linger as spores and germinate after about ten days, producing what looks like a return of the original infection. So the long prescription is really killing a possible second infection in advance with the first prescription, rather than reflecting how long it takes to resolve the symptomatic infection.
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Penicillin does not work on fungal infections. Chronic gas, persistent diarrhea, and similar symptoms that do not respond to penicillin point to something worse than a mild endotoxin problem, often fungal. Flowers of sulfur, fibrous foods, and adequate thyroid function are the way to handle intestinal fungus, since fungi convert sulfur into hydrogen sulfide that is locally toxic to themselves.
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Carrot salad, mushrooms, and other natural antibiotics produce similar hormonal effects without the drug. Two or three days of a daily raw carrot salad shifts cortisol, estrogen, and progesterone in the same direction as a course of penicillin. The disinfecting fibers in raw carrots and the antifungal effect of cooked button mushrooms do the same intestinal cleanup at the food level, and are the right place to start before reaching for an antibiotic.
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Penicillin VK can completely eliminate dairy intolerance in two weeks. A two-week course of penicillin VK eliminated milk allergy entirely in a Ray Peat community member who had varying success with thyroid, vitamin A, and aspirin alone. Milk maldigestion is generally a sign of bacterial overgrowth in the intestine, particularly small intestine bacterial overgrowth, and is not actually about the milk. Once the bacterial population is reduced, dairy is tolerated normally.
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Patients prepared with neomycin and a hefty dose of penicillin or amoxicillin before abdominal surgery become remarkably resilient. The standard pre-surgical protocol uses around 1 gram of neomycin daily for three days followed by a large penicillin or amoxicillin dose on the day of surgery, which sterilises the colon. Of these sterile-gut patients there are only one or two recorded deaths from surgery or its complications, and they almost never develop MRSA, sepsis, or cytokine storm. The colon repopulates within days after antibiotics are withdrawn.
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Long-term antibiotic use, including for Lyme disease, produces dramatic systemic improvement beyond the target infection. Patients on six-month antibiotic courses for Lyme often show better skin, better sleep, more energy, and clearer cognition even when their Lyme symptoms persist. This indicates the antibiotic is doing something useful by reducing the bacterial count and lowering endotoxin, independent of any specific pathogen it was prescribed for.
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Take vitamin K alongside any antibiotic course because gut bacteria normally produce it. Antibiotics deplete the bacteria that synthesise vitamin K, so supplementing K alongside the antibiotic prevents the bleeding risk associated with K deficiency. Recent research shows the K1 to K2 conversion can occur in animals and humans with strictly sterile guts, but vitamin K supplementation is still wise during antibiotic use because it is one of the best metabolic supplements available regardless and shares structural features with the tetracyclines.
Notable Quotes
"The old ones, erythromycin, penicillin, and tetracycline, they're still very effective."
[Ray Peat — Jodelle Fitzwater Podcast: Blood Tests, Hormones, Protein Intake & Ray's Carb List (June 2022)]
"Sometimes a third of the standard dose of penicillin or tetracycline or erythromycin will clear it (bowel infection) up."
[Ray Peat — KMUD: Allergy (March 2016)]
"Taking a series of penicillin doses or eating raw carrots, for example, can have very similar effects (keeping the intestine sterile). And in both rats and people, it turns out that anything that cleans up your intestine will lower your cortisol and estrogen and increase your progesterone because the intestine is such a chronic drain and source of stress."
[Ray Peat — Generative Energy #61: Bodybuilding and Steroids, Progesterone for Men, Basic Bioenergetic Therapies]
"If you are watching the symptoms of the infection, the first dose you take, like 250 milligrams of penicillin, you might not feel anything. Take another dose four hours later. And at some point, a second or third dose, you usually feel a sudden subsiding of the symptoms. And with the third or fourth dose, the symptoms of the infection are usually gone. But I think it's a good idea to taper off, take them maybe at eight hours interval for the next day or so."
[Ray Peat — Generative Energy #51: Heat Shock Proteins, Antibiotic Resistance, DHT Safety]
Important Things To Consider
Two-week, full-strength courses can sterilize the intestine and create fungal overgrowth. Heavy gigantic doses prescribed by doctors really do clear out the bacterial flora, but a sterile intestine that is no longer digesting properly is a good place for fungus to take root. If a long course is unavoidable, it is worth taking flowers of sulfur to keep fungus from filling the empty space.
Antibiotics can deplete vitamin K-producing gut bacteria. This can cause uncontrolled bleeding because clotting platelet function often depends on the vitamin K-dependent proteins from the liver. Aged cheese and well-cooked greens are ordinary food sources of vitamin K to keep in the diet as insurance. Supplementing Vitamin K is also an effective countermeasure.
Antibiotic resistance happens at the population level, not within one person. Hospitals consistently breed their own toxic resistant strains, but a single person using antibiotics occasionally does not develop resistance. The general "superbug" alarm has been blown out of proportion, and bacteriophage therapy (well developed in Eastern Europe and India) has produced cures in cases declared incurable in the West.
Food before drugs. Food that is easy to digest, with plenty of unirritating fiber, will normalize the flora in most cases without antibiotics at all. Cooked mushrooms and raw carrots are natural antibiotics that disinfect the gut. Penicillin and the other old antibiotics are appropriate when food alone is not enough.
Stop if a particular antibiotic produces a bad reaction. Some people react badly to specific molecules. The principle is to stop and try a different antibiotic family rather than push through.
Penicillin does nothing for fungal infections. If the suspected gut problem is fungal (chronic gas, persistent symptoms after multiple courses), penicillin will not help and may make things worse by clearing space for the fungus.
Carrot salad, activated charcoal, and bamboo shoots produce a milder version of the same antibacterial effect. For people who want to lower endotoxin without prescriptions, Ray Peat's carrot salad (raw grated carrot eaten daily) and activated charcoal capsules a few times per week reduce gut bacterial count and TNF-alpha levels. These are the over-the-counter approaches Georgi Dinkov recommends before reaching for an antibiotic.
Spore-based probiotics can have antibiotic effects. Ray Peat spoke about Bacillus Subtilis and Licheniformis having antibiotic-like effects and used them himself. These could be worth trying if someone is allergic or resistant to penicillin.