When Should You Not Do a 72 Hour Fast?

October 7, 2025 •

4 min read

According to a 2019 study, fasting for longer than 24 hours comes with an increased risk of complications, especially for certain individuals. Before attempting a 72 hour fast, it is critical to understand the specific circumstances, health conditions, and medications that make this practice unsafe for you.

Quick Summary

Extended fasting is not for everyone due to significant health risks. Understand the contraindications, medical conditions, and warning signs that mean you should avoid or stop a 72-hour fast, and always seek professional advice.

Key Points

Absolute Contraindications: Individuals who are pregnant, breastfeeding, underweight, have Type 1 diabetes, or a history of eating disorders should never attempt a 72-hour fast.
Medical Supervision is Critical: A 72-hour fast is a medical intervention and requires clearance from a doctor, especially for those with pre-existing conditions like heart disease or kidney issues.
Watch for Immediate Warning Signs: Symptoms such as severe dizziness, chest pain, confusion, or persistent vomiting require you to break your fast and seek medical help immediately.
Be Cautious with Medications: Many medications, including insulin and certain blood pressure drugs, require careful monitoring and adjustment during a fast, making medical oversight essential.
Refeeding Syndrome Risk: Reintroducing food too quickly after a prolonged fast can cause life-threatening electrolyte shifts, particularly for those who are malnourished.
Understand the Initial Discomfort: Mild headaches and fatigue are common in the first 48 hours, but severe or worsening symptoms are a red flag to stop.
Consider Safer Alternatives: For most people, less extreme fasting methods like intermittent fasting (e.g., 16:8) can offer metabolic benefits with dramatically lower risks.

Who Should Never Attempt a 72 Hour Fast?

While some healthy adults may undertake a 72-hour fast under medical supervision, several groups should never attempt prolonged fasting due to severe health risks. These are not recommendations to be taken lightly; they are absolute contraindications designed to prevent serious complications.

Pregnant or Breastfeeding Women: Both pregnancy and breastfeeding place immense nutritional demands on the body. Fasting can deprive both mother and child of essential calories, vitamins, and minerals, risking developmental issues for the fetus or impacting milk supply for the baby.
Individuals with Type 1 Diabetes: People with type 1 diabetes, and some with type 2 diabetes on certain medications, are at a very high risk of dangerous hypoglycemia (dangerously low blood sugar) during a prolonged fast. The risk of diabetic ketoacidosis, a life-threatening condition, also increases significantly.
People with a History of Eating Disorders: Fasting can trigger or exacerbate disordered eating behaviors, making it a dangerous and inappropriate practice for anyone with a history of anorexia, bulimia, or binge eating disorder.
Those who are Underweight: People with a BMI below 18.5 lack the necessary body fat stores to sustain a prolonged fast safely. Fasting in this state can lead to dangerous metabolic disruptions and excessive muscle protein breakdown.
Individuals with Advanced Organ Disease: Severe or advanced liver or kidney disease impairs the body’s ability to process and eliminate waste, making fasting potentially dangerous. The stress of a prolonged fast can overwhelm these compromised organs.

Medical Conditions Requiring Extreme Caution and Supervision

Beyond the groups who should avoid fasting entirely, others must proceed with extreme caution and require strict medical supervision before considering a 72-hour fast. This applies particularly to those over 65, those with a high BMI, and anyone with pre-existing heart or metabolic conditions.

Comparison of Fasting Risk Factors


Medical Condition	Risk Level During 72-Hour Fast	Primary Reason for Risk	Action Required
Type 1 Diabetes	Extreme	Risk of hypoglycemia and diabetic ketoacidosis.	Absolutely avoid without explicit, careful medical supervision and medication adjustment.
Chronic Kidney Disease	High	Can worsen kidney function due to metabolic stress.	Always consult a nephrologist; often contraindicated.
Heart Conditions	High	Risk of cardiac arrhythmias and electrolyte imbalance.	Mandatory medical clearance and monitoring.
History of Eating Disorder	Extreme	Triggers unhealthy psychological patterns.	Avoid entirely; seek mental health support.
Underweight (BMI < 18.5)	Extreme	Insufficient fat stores, risks severe muscle loss.	Avoid entirely; focus on healthy weight gain.
Pregnancy/Breastfeeding	Extreme	Insufficient nutrients for mother and child.	Avoid entirely.
Prescription Medications	High (variable)	Medication may require food or affect blood sugar/pressure.	Mandatory medical consultation for all medication adjustments.
Advanced Liver Disease	High	Impaired metabolic and detoxification functions.	Mandatory medical clearance; often contraindicated.

When to Stop Your Fast Immediately

It is imperative to listen to your body and recognize serious warning signs that require you to break your fast immediately and seek medical attention. Ignoring these can lead to life-threatening complications.

Warning Signs to Terminate a Fast:

Severe dizziness or fainting: Can signal dangerously low blood pressure (orthostatic hypotension) or severe hypoglycemia.
Chest pain or irregular heartbeat: May indicate a cardiac arrhythmia triggered by electrolyte imbalances, particularly low potassium.
Persistent vomiting or diarrhea: Rapid fluid and electrolyte loss can lead to severe dehydration and dangerous imbalances.
Confusion or disorientation: Cognitive impairment is a sign of extreme hypoglycemia or severe electrolyte issues.
Extreme muscle cramps or weakness: Can signal dangerous electrolyte shifts, especially low potassium and phosphate levels.
Fruity-scented breath or labored breathing: These are symptoms of ketoacidosis, a medical emergency that can occur in some individuals.
Inability to function normally: If basic daily activities feel impossible due to fatigue or other symptoms, terminate the fast.

The Dangers of Refeeding Syndrome

After a prolonged fast, especially one over 48 hours, the body is highly susceptible to a condition called refeeding syndrome. This potentially fatal condition occurs when food is reintroduced too quickly, causing dangerous shifts in fluid and electrolyte levels. The risk is highest for those who are underweight or malnourished before the fast. Safe refeeding involves a slow, careful reintroduction of small meals over several days, emphasizing gentle, easily digestible foods. The initial meal post-fast should be small and low in carbohydrates to avoid a sudden insulin spike.

The Critical Importance of Medical Supervision

Ultimately, a 72-hour fast is a significant medical intervention, not a casual lifestyle choice. For healthy individuals, a doctor's consultation can confirm fitness, adjust medications, and establish a safety plan. For anyone with pre-existing health conditions, this supervision is non-negotiable and often results in a recommendation against extended fasting entirely. The potential health risks of ignoring contraindications far outweigh any perceived benefits. Prioritizing safety and working with a healthcare professional is the only responsible way to approach a 72-hour fast, if at all. For those seeking safer alternatives, time-restricted eating methods like 16:8 can offer similar metabolic benefits with far lower risk.

Conclusion: Prioritize Safety Above All Else

While extended fasting has garnered significant attention for its potential benefits, it is not a suitable practice for everyone and carries significant risks for specific populations. The decision to undertake a 72 hour fast should be based on a thorough understanding of one's personal health status and a candid discussion with a healthcare provider. Individuals who are pregnant or breastfeeding, have diabetes, a history of eating disorders, or are underweight must avoid this practice entirely. For others with existing health conditions, medical clearance and supervision are non-negotiable. Recognizing the immediate warning signs to terminate a fast and understanding the dangers of refeeding syndrome are crucial safety measures. Always prioritize safety over the pursuit of potential benefits, and remember that safer, less extreme alternatives often exist for improving metabolic health. For more general information on safe fasting practices, consult reputable health organizations.

Frequently Asked Questions

You should not attempt a 72-hour fast if you are taking any prescription medication without first consulting a doctor. Certain medications, such as insulin, blood pressure drugs, and others, require careful dosage adjustment or must be taken with food, making fasting potentially dangerous.

No, it is not safe for pregnant or breastfeeding women to do a 72-hour fast. The body requires a steady supply of nutrients and calories during these periods to support both the mother's health and the baby's development.

Prolonged fasting can be extremely dangerous for individuals with a history of eating disorders, as it can trigger a relapse into unhealthy and disordered eating patterns. Psychological and physical risks are significant, and it should be avoided entirely.

If you experience severe dizziness, lightheadedness, or feel like you might faint, you should stop the fast immediately. These symptoms can indicate serious electrolyte imbalances or orthostatic hypotension, which require immediate attention.

For those with Type 1 diabetes, a 72-hour fast is an absolute contraindication due to the high risk of dangerous hypoglycemia and diabetic ketoacidosis. For Type 2 diabetics, fasting is only recommended with strict medical supervision and requires careful medication adjustment.

People with heart conditions should not attempt a 72-hour fast without explicit medical clearance and supervision. Prolonged fasting can cause electrolyte shifts that lead to cardiac arrhythmias, a potentially life-threatening complication.

To safely break a 72-hour fast, reintroduce food slowly with small, easily digestible meals. Begin with bone broth or small amounts of fermented foods, and avoid large, high-carbohydrate, or processed meals for the first 24-48 hours to prevent refeeding syndrome.

Persistent vomiting is a serious warning sign that requires you to break the fast immediately and seek medical attention. It leads to rapid fluid and electrolyte depletion, which can be dangerous.