Health class and the nurse’s office handed down rules that felt medical: wait an hour to swim, sugar makes kids wild, eight glasses a day, cracking knuckles ruins your joints. Plenty of that advice came from habit, posters, and half-remembered studies. Here are the school-adjacent health claims we track—and what the evidence supports now.
COVID-19 vaccines cause blood clots or increase stroke risk
What we know now
While rare side effects like specific blood clot disorders were linked to certain COVID-19 vaccines (e.g., Johnson & Johnson, which is no longer available in the U.S.), the vast majority of COVID-19 vaccines do not cause blood clots or increase stroke risk. The benefits of vaccination far outweigh these rare risks, and the risk of blood clots and heart problems is higher with COVID-19 infection itself.
COVID-19 vaccines cause cancer or more aggressive cancer
What we know now
This myth is false. Researchers have found no evidence linking COVID-19 vaccines to an increase in cancer rates or more aggressive forms of cancer. This misconception often arises from individual experiences or misinterpretations of data.
This myth is false. The virus that causes COVID-19 spreads from person to person regardless of temperature. People in all climates, both hot and cold, have contracted COVID-19.
This is a false claim. mRNA vaccines (like Pfizer and Moderna) deliver instructions to your cells to make a protein from the virus, triggering an immune response. The mRNA does not enter the nucleus of the cell where your DNA is stored and therefore cannot alter your DNA. The instructions are broken down and removed by the body.
Dietary supplements or alcohol can prevent or treat COVID-19
What we know now
There is no scientific evidence to support the claim that dietary supplements, specific foods (like hot peppers or garlic), or alcoholic beverages can prevent or treat COVID-19. Relying on such methods can be dangerous and delay appropriate medical care.
Flu shots cannot give you influenza. Needle vaccines use inactivated virus or only part of one flu gene, neither of which can cause flu illness. Feeling sore or tired afterward is usually a mild immune response, not the flu, though you can still catch influenza or another virus later through ordinary exposure.
Waking a sleepwalker is dangerous and can cause a heart attack or brain damage
What we know now
Waking a sleepwalker does not cause a heart attack, brain damage, or death; neurologists describe those claims as myth. Sleepwalking (somnambulism) is a partial-arousal disorder, usually in deep non-REM sleep early in the night. Abrupt waking can produce temporary confusion or a startle response that might lead to falls or thrashing, so gently guiding the person back to bed is preferred when possible—but the danger is incidental injury and agitation, not catastrophic medical harm.
The hymen is a completely closed membrane broken by first intercourse
What we know now
The hymen is usually a thin ring of tissue around the vaginal opening, not a complete seal. It normally has an opening for menstrual flow, can change through many ordinary activities, and is not a reliable indicator of sexual history or virginity. A fully closed hymen (imperforate hymen) is rare and is a medical condition, not typical anatomy.
Moderate coffee intake does not dehydrate healthy adults who drink it regularly. Caffeine can briefly increase urine output, especially in large doses or after caffeine abstinence, but the water in coffee offsets that effect at typical intakes (roughly a few cups a day). A 2014 PLOS One crossover study found no difference in hydration markers between moderate coffee and water in men who already drank coffee daily. Earlier reviews reached the same broad conclusion for ordinary lifestyle caffeine use. Water is still the simplest hydration choice; coffee can still count toward daily fluid.
The food pyramid was a dietary guideline that emphasized a balanced diet of carbohydrates, proteins, and fats. However, it was later revised to focus on a more plant-based diet with a greater emphasis on fruits, vegetables, and whole grains.
Drinking coffee or caffeinated beverages will stunt your growth
What we know now
Reviews of the evidence have not found a credible link between moderate caffeine intake and reduced adult height. Most final height is determined before teens commonly start drinking coffee regularly.
The 1998 Lancet paper linking MMR to autism was retracted as fraudulent. Large cohort studies (including Danish nationwide studies of more than 500,000 and 650,000 children), meta-analyses, and reviews by WHO and the Institute of Medicine find no increased autism risk after MMR, including in children with higher baseline risk.
Eggs are extremely bad for cholesterol and should be avoided
What we know now
Studies show eggs have minimal impact on blood cholesterol for most people and provide valuable nutrients. The American Heart Association (AHA) dropped its specific recommendation to limit eggs in 2002. Since then, multiple studies have shown that eating eggs—even up to 12 per week—does not significantly impact cholesterol levels or increase cardiovascular risk in most people, including those at higher risk for heart disease.
Everyone should drink eight 8-ounce glasses of water per day
What we know now
There is no universal hydration rule that fits everyone. Most people can rely on thirst and normal food and beverage intake, because needs vary with activity, climate, and individual health.
The human eye is designed to adapt to different lighting conditions. While reading in dim light can cause temporary eye strain, it does not cause lasting damage to your vision, a fact widely accepted and publicized by the early 2000s.
Breastfeeding is now recognized as the optimal form of infant nutrition, offering immune protection, better developmental outcomes, and long-term health benefits for both infants and mothers. WHO guidelines highlight the importance of breastfeeding, especially exclusive breastfeeding for the first six months of life though formula is still vital for some families.
The idea of a "sugar high" gained popularity in the 1970s but sugar does not cause hyperactivity in children. The relationship between sugar intake and behavior is complex and influenced by factors like blood sugar levels, genetics, and environmental factors.
In the early years of the AIDS epidemic in the 1980s, it was often mistakenly perceived as a disease exclusive to gay men. It was later understood that AIDS is caused by the HIV virus, which can affect anyone regardless of sexual orientation, and is transmitted through specific bodily fluids.
Crack Cocaine Was Instantly Addictive After One Use
What we know now
While crack cocaine is highly addictive, the idea that it was instantly addictive after a single use was a widespread misconception, often fueled by sensationalized media and political rhetoric during the 1980s. Addiction risk is high but not universally true for everyone. Addiction is a complex process influenced by multiple factors.
While saturated and trans fats are harmful, research has clarified the benefits of monounsaturated and polyunsaturated fats (e.g., omega-3 fatty acids). These "good fats" help reduce inflammation, improve cholesterol levels, and support overall cardiovascular health.
Cracking your knuckles causes arthritis later in life
What we know now
Clinical studies have found no meaningful link between habitual knuckle cracking and arthritis. The popping sound comes from gas bubbles in joint fluid, not from damaging the joints themselves.
The origins of this myth are unclear but it possibly arose as a way for parents and teachers to discourage children from swallowing gum and other non-food items. Medical professionals and scientific sources have consistently debunked this myth. Gum is digested by the body within 24-48 hours.
You must wait at least an hour after eating before swimming
What we know now
There is no good evidence that swimming right after a meal increases cramping or drowning risk. Digestion may divert some blood flow, but not enough to prevent normal swimming for healthy people.
Alcohol can make your skin feel warm by dilating blood vessels near the surface, but it increases heat loss and can lower core body temperature. It can raise hypothermia risk even when it feels comforting.
Colds are caused by viruses, not by being chilled or having damp hair. Cold weather may keep people indoors together, but infection still requires exposure to a pathogen.
Heat loss depends on exposed surface area, not a special rule for the head. The head only loses disproportionate heat when it is the main uncovered part of the body.
Shaving does not make hair grow back thicker, darker, or faster. A razor only cuts the hair shaft at the surface and leaves a blunt tip, which can feel stubbly or look darker until the hair tapers again; it does not change the follicle. A 1970 Journal of Investigative Dermatology study comparing repeatedly shaved legs with unshaved controls found no significant differences in hair width, coarseness, or growth rate—consistent with what dermatologists still report.
Eating a lot of carrots will give you excellent night vision
What we know now
Carrots provide vitamin A, which helps prevent deficiency-related vision problems, but they do not give healthy people superhuman night vision. The myth was amplified by World War II propaganda.
