Medical myths can persist in clinical practice, even after evidence has challenged them for years. From preparation for colonoscopy and surgery to antibiotic use, wound care, and vitamin D screening, several familiar recommendations may not reflect the current evidence. Here are 10 persistent medical myths and evidence.

1. Strict Liquid Diet Before Colonoscopy

Myth: Before colonoscopy, patients must follow a clear-liquid diet for 1-2 days to adequately cleanse the colon.

Evidence: A low-residue diet on the day before colonoscopy provides bowel cleansing comparable to that achieved with a clear-liquid diet (odds ratio [OR], 0.96; 95% CI, 0.72-1.29) and does not reduce the adenoma detection rate (OR, 1.03; 95% CI, 0.86-1.23). A systematic review and meta-analysis found that patients following a low-residue diet experienced less hunger (OR, 0.36) and nausea (OR, 0.72) and were more willing to undergo repeat colonoscopy (OR, 2.23). Another meta-analysis reported similar findings.

In addition to the type of diet, timely administration of bowel preparation in divided doses, known as split dosing, is universally considered the clinical standard of care for optimizing colonoscopy bowel preparation quality.

2. Alcohol During Antibiotic Treatment

Myth: Alcohol should never be consumed while taking antibiotics. Moderate alcohol consumption while taking metronidazole inevitably causes a severe disulfiram-like reaction.

Evidence: For most commonly used antibiotics, including penicillins, cephalosporins, and macrolides, moderate alcohol consumption does not produce clinically relevant pharmacokinetic or toxic interactions. A systematic review found that true disulfiram-like reactions caused by the inhibition of aldehyde dehydrogenase are primarily associated with a small number of cephalosporins containing an N-methylthiotetrazole side chain, such as cefotetan and cefoperazone. However, evidence regarding cotrimoxazole is limited. For metronidazole, controlled human studies showed no toxic blood acetaldehyde spikes. However, alcohol may worsen gastrointestinal adverse effects, such as nausea, vomiting, and dizziness, and case reports of these reactions have been published. A previous study found no evidence of a disulfiram-like reaction between metronidazoles and ethanol.

A 2023 case-control study also evaluated the potential interaction. For this reason, prescribing information for metronidazole continues to recommend abstinence from alcohol to prevent a disulfiram-like reaction.

3. Fasting Before Surgery

Myth: Before elective surgery, patients should begin strict fasting at midnight and must not consume liquids for 2 hours before anesthesia.

Evidence: Guidelines from professional societies, including the European Society of Anesthesiology and Intensive Care, recommend that healthy adults may consume clear liquids, including water, tea or coffee without milk, and clear juices, up to 2 hours before inducing anesthesia. Solid foods should generally be avoided for 6 hours before anesthesia. These recommendations were established in perioperative guidelines from the European Society of Anesthesiology and have been further addressed in updated guidance.

Allowing clear liquids closer to surgery can reduce thirst, discomfort, and dehydration. However, in practice, delays in surgical schedules can result in unnecessarily prolonged fasting. Newer approaches, including “sip til send,” allow patients to continue sipping clear liquids until they are called for surgery.

These approaches are being evaluated and incorporated into local protocols. Studies using abdominal ultrasound have shown low gastric volumes without an observed increase in aspiration incidence in clinical practice. A 2024 quality improvement initiative also evaluated strategies to improve adherence to perioperative fasting recommendations.

4. Wound Healing

Myth: Wounds heal faster and best when left open to the air so they can form a dry scab

Evidence: A review highlighted that maintaining a moist wound environment prevents cell death caused by desiccation and supports keratinocyte migration, angiogenesis, and collagen synthesis. Thick dry scabs can mechanically delay reepithelialization. Another review summarized wound care management and found that adequate wound cleansing helps protect against contamination, reduces pain during dressing changes, and promotes a healthy wound bed in both acute and chronic wounds.

5. Oral Iron Supplementation

Myth: Patients with iron deficiency anemia should take high doses of oral iron one to three times a day to replenish iron stores as quickly as possible.

Evidence: Oral iron doses of 60-100 mg or higher spike serum hepcidin, a liver hormone regulating iron balance for 24-48 hours, blocks the gut from absorbing extra iron. This can reduce the fractional absorption of the subsequent doses administered the following day.

Alternate-day dosing avoids this pronounced hepcidin response and increases fractional iron absorption and improves gastrointestinal tolerability while maintaining comparable or adequate iron replenishment over time.

6. Sunscreen Application

Myth: Chemical sunscreens must be applied 20-30 minutes before sun exposure because ultraviolet (UV) filters require time to activate in the skin.

Evidence: Chemical UV filters begin to absorb UV radiation after application and do not require a chemical or physical activation period. The recommendation to apply sunscreen 15-30 minutes before sun exposure is based primarily on the time needed for the product to dry and form a homogeneous, mechanically stable protective film on the skin.

Protection may be compromised sooner if the product is exposed to water or mechanically disrupted by clothing before the film is adequately set.

7. Showering After Sutures

Myth: Surgical wounds closed with sutures or staples must not come into contact with water until the sutures are removed, usually after 10-14 days.

Evidence: For clean, primarily closed surgical wounds without drains, a Cochrane review of 857 patients found no statistically significant increase in wound infection risk with careful showering within 24-48 hours (relative risk, 0.96; 95% CI, 0.62-1.48).

Brief showers with plain tap water or mild soap are appropriate, provided that the wound is not scrubbed. Prolonged soaking and the use of full baths should be avoided. Greater caution is required in patients with impaired wound healing or surgical drains. A systematic review and meta-analysis found that postoperative showering after primary closure does not increase the risk for surgical site infections or wound complications. The available evidence applies primarily to smaller wounds and has not yet been established for larger surgical wounds.

8. Vitamin D Screening

Myth: Vitamin D levels should be routinely screened in all adults, and supplementation should be prescribed when laboratory results are low.

Evidence: The US Preventive Services Task Force has issued an “I statement” for population-wide screening for vitamin D deficiency in asymptomatic, nonpregnant adults, indicating that current evidence cannot prove whether benefits outweigh risks.

Large intervention studies in generally healthy populations have not shown significant reductions in overall mortality, cardiovascular morbidity, or fractures with routine screening and supplementation in the absence of specific indications. Testing is generally reserved for patients with relevant clinical indications or those at increased risk, including individuals with osteoporosis, malabsorption, or residence in nursing homes.

9. Sputum Color

Myth: Green or yellow sputum associated with an acute cough indicates a bacterial infection that requires antibiotic treatment.

Evidence: Green or yellow sputum is primarily caused by the enzyme myeloperoxidase, which is produced by neutrophils and reflects the local inflammatory response. Both viral and bacterial infections can cause this discoloration.

Although colored sputum is statistically associated with bacterial infections, its diagnostic specificity is low. With a sensitivity of approximately 79% and specificity of approximately 46%, sputum color alone cannot reliably distinguish bacterial from viral infections and does not, by itself, justify antibiotic treatment. A prescribing study also evaluated antibiotic prescriptions for discolored sputum. A systematic review and meta-analysis concluded that sputum color can help identify bacterial infection during acute exacerbations of chronic obstructive pulmonary disease but is not sufficiently accurate to be used as a standalone diagnostic marker.

10. An Apple a Day

Myth: Eating an apple every day keeps the doctor away. The proverb “an apple a day keeps the doctor away” dates back to an 1866 Welsh folk rhyme promoting healthy habits, although science suggests that the saying is metaphorical rather than a literal medical rule.

Evidence: A cross-sectional study using National Health and Nutrition Examination Survey data found no statistically significant association, after multivariable adjustment, between daily apple consumption and having ≥ one physician visit per year (OR, 1.19; 95% CI, 0.93-1.53). A slight association was found between avoiding prescription medications (OR, 1.27).

The findings were primarily interpreted as reflecting a general “healthy user effect.” A systematic review and dose-response meta-analysis of prospective studies found an inverse association between high fruit and vegetable intake and overall mortality and cardiovascular disease.

What Medical Myths Have You Encountered?

What medical myths have you encountered in your practice? Share your experiences in the comments.

This story was translated from Coliquio, part of the Medscape Professional Network.