Magnesium citrate colonoscopy prep is an osmotic laxative used to cleanse the bowel before endoscopic examination. In the UK, it is available as Citramag or in combination products such as Picolax (sodium picosulfate with magnesium citrate). The preparation works by drawing water into the intestines, softening stool and stimulating bowel movements to ensure clear visualisation during colonoscopy. Whilst polyethylene glycol (PEG)-based preparations are more commonly used in UK practice, magnesium citrate remains an important option for selected patients. Proper administration and adherence to your endoscopy unit's specific instructions are essential for safe, effective bowel cleansing and procedural success.

What Is Magnesium Citrate and How Does It Work for Bowel Preparation?

Magnesium citrate is an osmotic laxative that has been used for bowel cleansing prior to colonoscopy and other lower gastrointestinal procedures. It works by drawing water into the intestinal lumen through osmosis, which softens the stool and stimulates peristalsis—the wave-like muscle contractions that propel contents through the digestive tract. This dual action results in increased bowel movements and thorough evacuation of the colon, which is essential for clear visualisation during endoscopic examination.

The compound consists of magnesium bound to citric acid. Its laxative effect relies on limited absorption in the gastrointestinal tract, where it increases osmotic pressure, preventing water reabsorption and promoting fluid retention within the bowel. This mechanism typically produces bowel movements within several hours after ingestion, though individual response times vary.

Important context for UK practice: Magnesium citrate is used in UK practice for colonoscopy preparation, though less commonly as a sole agent than polyethylene glycol (PEG)-based preparations. UK practice is guided by British Society of Gastroenterology (BSG) and European Society of Gastrointestinal Endoscopy (ESGE) guidelines, along with MHRA safety advisories.

In the UK, magnesium citrate is available as Citramag, while combination products containing sodium picosulfate with magnesium citrate (such as Picolax) are also commonly used. The selection of bowel preparation depends on patient factors, local protocols, and individual risk assessment.

If you have been prescribed magnesium citrate or a combination product containing it, it is essential to follow the specific instructions provided by your endoscopy unit, as preparation protocols vary between hospitals and clinical situations.

How to Take Magnesium Citrate Before a Colonoscopy

If magnesium citrate has been prescribed for your colonoscopy preparation, it is crucial to follow the specific instructions provided by your hospital or endoscopy unit. Never substitute generic advice for the exact protocol you've been given.

Typical administration guidance (though always defer to your hospital's protocol):

• Timing: Usually commenced the afternoon or evening before the colonoscopy, with the last dose completed at least 4–6 hours before the procedure

• Dosage: Follow the exact dosing instructions for your specific product (e.g., Citramag or Picolax sachets)

• Hydration: Drink plenty of clear fluids (water, black tea or coffee without milk, clear soup/stock) throughout the preparation period as directed

• Temperature: Chilling the solution may improve palatability

• Dietary restrictions: Follow a low-fibre diet as directed, typically starting 24–48 hours before the procedure

It is essential to remain near toilet facilities once you begin the preparation, as bowel movements will be frequent and urgent. The goal is to achieve clear or pale yellow liquid stools—resembling urine in consistency—which indicates adequate bowel cleansing.

Practical considerations:

• Avoid red or purple drinks as they can stain the bowel

• Oral iron supplements are often stopped several days before the procedure

• If you are on fluid restriction due to heart or kidney conditions, seek specific advice before increasing your fluid intake

Medication considerations: Inform your endoscopy team about all medications you take. Some drugs, particularly those affecting electrolyte balance (diuretics, ACE inhibitors) or those requiring consistent absorption (anticoagulants, antiepileptics, contraceptives), may need dose adjustment or temporary discontinuation. If you experience vomiting or severe diarrhoea while taking oral contraceptives, additional contraceptive precautions may be needed. Never stop prescribed medications without medical advice.

If you experience severe abdominal pain, vomiting that prevents you from completing the preparation, or no bowel movements within several hours of taking the preparation, contact your endoscopy unit or seek medical advice promptly.

Side Effects and Safety Considerations

Magnesium citrate, like all bowel preparations, can cause side effects ranging from mild discomfort to potentially serious complications, particularly in vulnerable patient groups. Understanding these risks is essential for safe use.

Common side effects include:

• Abdominal cramping, bloating, and discomfort

• Nausea and occasionally vomiting

• Perianal irritation from frequent bowel movements

• Dizziness or light-headedness due to fluid shifts

• Electrolyte disturbances, particularly low potassium, sodium, or calcium levels

Serious safety concerns relate primarily to magnesium accumulation and electrolyte imbalance. Magnesium citrate is contraindicated in patients with:

• Intestinal obstruction or perforation

• Ileus (lack of normal intestinal motility)

• Severe acute inflammatory bowel disease or toxic megacolon

• Significant renal impairment: Reduced kidney function prevents normal magnesium excretion, risking hypermagnesaemia (elevated magnesium levels), which can cause muscle weakness, cardiac arrhythmias, respiratory depression, and altered consciousness

• Severe dehydration

• Heart block or other significant cardiac conduction abnormalities

The MHRA has issued safety warnings regarding all bowel cleansing solutions and the risk of serious fluid and electrolyte disturbances, emphasising the importance of adequate hydration and careful patient selection. Patients should be advised to maintain fluid intake before, during, and after the preparation unless otherwise directed.

When to seek immediate medical attention:

• Severe or persistent vomiting preventing fluid intake

• Signs of dehydration (reduced urination, extreme thirst, confusion)

• Chest pain, palpitations, or irregular heartbeat

• Severe abdominal pain or distension

• Rectal bleeding beyond minor spotting

• Symptoms of hypermagnesaemia: muscle weakness, difficulty breathing, profound drowsiness

• Symptoms of hyponatraemia: confusion, headache, seizures (particularly in patients taking diuretics or certain antidepressants)

Pregnant or breastfeeding women should only use magnesium citrate under direct medical supervision, as safety data in these populations are limited.

If you experience any suspected adverse reactions, report them via the MHRA Yellow Card scheme.

Alternative Bowel Preparation Options in the UK

In the United Kingdom, several bowel preparation options are available, with selection guided by BSG/ESGE recommendations, patient factors, and local protocols.

Standard UK bowel preparation options include:

• Moviprep: A PEG-based solution with added electrolytes and ascorbic acid, typically taken as two litres split into doses the day before and morning of the procedure

• Klean-Prep (Macrogol): A four-litre PEG solution, often split-dosed for improved tolerability

• Citramag (magnesium citrate): Used in some UK protocols, particularly for patients who cannot tolerate larger volumes

• Picolax/Citrafleet (sodium picosulfate with magnesium citrate): A stimulant laxative combined with osmotic agent, commonly used in UK practice

• Low-volume preparations: Such as Plenvu (PEG-based) or Izinova (sulphate-based), which require less fluid intake but must be accompanied by additional clear fluids

Polyethylene glycol (PEG)-based preparations are generally preferred for patients with renal impairment, heart failure, or at risk of electrolyte disturbances due to their isotonic nature. Oral sodium phosphate solutions are generally avoided due to risks of acute kidney injury and electrolyte disturbances, as highlighted in MHRA safety advisories.

The choice of preparation depends on multiple factors including patient comorbidities, renal function, cardiac status, previous tolerance of bowel preparations, and the endoscopist's preference. Split-dose regimens—where preparation is divided between the day before and the morning of the procedure—have demonstrated superior bowel cleansing and improved adenoma detection rates compared to single-day preparations.

For patients unable to tolerate oral preparations due to swallowing difficulties, severe nausea, or other contraindications, nasogastric tube administration may be considered in hospital settings.

Patient-centred considerations:

Your endoscopy unit should provide clear written and verbal instructions tailored to your specific preparation. If you have concerns about the prescribed regimen—particularly if you have kidney disease, heart conditions, or take multiple medications—discuss alternatives with your gastroenterology team well in advance of your procedure.

Adequate bowel preparation is crucial for procedural success; poor preparation may result in missed pathology, cancelled procedures, or the need for repeat colonoscopy. If you are struggling to complete your preparation or have questions about the process, contact your endoscopy unit promptly rather than abandoning the regimen. They can provide guidance, reassurance, or arrange alternative strategies to ensure your colonoscopy proceeds safely and effectively.

Frequently Asked Questions