Contraindications and side effects of the ketogenic diet

The ketogenic diet changes, in an important way, how the body produces energy — and that change isn't neutral for everyone. This guide isn't meant to alarm you: it's meant to help you know who should be followed by a professional, what can happen, and when a symptom deserves medical attention.

Who should talk to a doctor before starting

For these situations, starting keto without supervision isn't a good idea:

Diabetes, especially type 1, or treatment with insulin, sulfonylureas or SGLT2 inhibitors: sharply cutting carbs can change the risk of hypoglycemia or ketoacidosis. Treatment needs to be assessed by your care team before starting, and shouldn't be adjusted on your own.
Pregnancy and breastfeeding: nutritional needs change and should be followed by a professional, not managed with a generic guide.
Kidney disease, especially if advanced: protein, fluid and electrolyte intake, and any medication, need an individual assessment.
Advanced liver disease: the ability to handle a major metabolic and nutritional change needs to be assessed by a specialist.
Current or past pancreatitis and other pancreatic or gallbladder conditions: a very high-fat diet may need a specific assessment.
Eating disorders, current or past: a restrictive eating pattern can interfere with that recovery.
Children and teenagers: a therapeutic form of keto exists in medical settings (for example for drug-resistant epilepsy), but it's managed by a specialist — that's not what this guide is about.
Other medications whose dosage depends on diet (for example anticoagulants like warfarin, or blood pressure medication): a major dietary change may require an adjustment, to discuss with whoever prescribed them.

If you recognize yourself in one of these points, the step before any other decision is talking to a doctor.

Possible side effects and what to watch for

It's worth distinguishing between symptoms that can appear in the initial phase, changes that deserve monitoring over time, and risks or signs that require an evaluation.

Initial symptoms, often transitory

The keto flu of the first days.
Constipation, which can be favored by changes in the amount and type of fiber and by insufficient fluid intake.
Breath with a distinctive smell (due to ketones, in particular acetone).
Muscle cramps, which can be associated with changes in fluids or electrolytes (see also 5 beginner mistakes on keto), but can also have other causes.

Changes to monitor

Menstrual cycle changes have been reported by some people, but can have many causes and deserve an evaluation if they persist.
Increased LDL cholesterol in some people: if you already have a known cardiovascular risk, it's a value to monitor with your doctor, not to ignore or automatically fear.

Risks worth knowing about

The risk of kidney stones may increase, especially with therapeutic ketogenic diets followed long-term. Anyone who has already had kidney stones should talk to their doctor; proper hydration remains important.

Some mild effects can be temporary, but shouldn't automatically be considered normal. If a symptom is intense, persists, worsens or interferes with daily activities, it's worth talking to a healthcare professional.

Signs that require urgent medical attention

Some signs shouldn't be managed by waiting for them to pass:

persistent, significant pain in the side, back or abdomen
signs of marked dehydration, such as intense dizziness, confusion or a fast heartbeat
palpitations or an irregular heartbeat
persistent vomiting or inability to drink
severe weakness, rapid or difficult breathing, or marked changes in mental state

If nausea or vomiting are accompanied by abdominal pain, severe weakness, rapid or difficult breathing, confusion or marked malaise, urgent medical attention is needed, especially if you have diabetes or are on SGLT2 inhibitor therapy. Ketoacidosis is different from nutritional ketosis and can, in some cases, occur even without very high blood glucose values.

In these cases the right thing to do is seek urgent medical attention, not another article.

On medication: one single rule

If you take medication for a chronic condition, especially for diabetes or blood pressure, tell whoever prescribed it before starting keto. A major dietary change can change how much of that medication you need, and doses are reviewed by whoever manages your care, not on your own and not based on what you read online — this guide included.

Sources

This article is for informational purposes only and does not replace advice from a doctor or nutritionist. If you recognize yourself in one of the situations described, talk to a professional before starting the ketogenic diet — even if you're already following our guide to getting started.