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Understanding Orlistat and Constipation: Clinical Evidence

Published on: June 03, 2026

Many UK patients ask can orlistat make you constipated before starting treatment. While orlistat is primarily associated with loose stools and oily spotting due to its fat-blocking mechanism, constipation can occasionally occur, particularly when patients drastically reduce dietary fat intake. At Cured Pharmacy, our clinical team helps patients understand orlistat's digestive effects and provides genuine UK-licensed orlistat from £32.00 with next day delivery across the UK.

How Orlistat Affects Your Digestive System

Orlistat works by inhibiting pancreatic and gastric lipases, the enzymes responsible for breaking down dietary fat in your intestine [1]. By blocking approximately 25-30% of dietary fat absorption, orlistat prevents these calories from being absorbed, instead allowing undigested fat to pass through your digestive system [1]. This mechanism is why the most commonly reported side effects involve oily stools, increased bowel movements, and faecal urgency rather than constipation.

However, some patients experience constipation when taking orlistat, typically for two reasons. First, if you reduce your fat intake too drastically in an attempt to avoid gastrointestinal side effects, you may inadvertently decrease overall food volume and fibre intake, which can slow bowel transit time [2]. Second, inadequate fluid intake combined with dietary changes can contribute to harder stools and difficulty passing them.

Can Orlistat Make You Constipated? Clinical Evidence

In the landmark XENDOS trial involving over 3,000 patients taking orlistat 120mg three times daily, constipation was reported in approximately 5.2% of participants, compared to 4.5% in the placebo group — a statistically insignificant difference [3]. This suggests that when constipation occurs during orlistat treatment, it's often related to dietary changes rather than the medication itself.

The most common gastrointestinal effects reported in clinical trials were oily spotting (26.6%), flatus with discharge (23.9%), faecal urgency (22.1%), and fatty or oily stools (20.0%) [3]. These effects typically occur when patients consume meals containing more than 30% of calories from fat, as the unabsorbed fat passes through the digestive system.

If you're experiencing constipation while taking orlistat, it's worth reviewing your dietary fibre intake. The MHRA recommends maintaining a balanced diet with adequate fibre and fluid intake throughout orlistat treatment to support normal digestive function [4].

Managing Digestive Side Effects

Our superintendent pharmacist Tarun Kumar advises patients to maintain a diet with approximately 30% of calories from fat, distributed evenly across three main meals. This approach minimises gastrointestinal side effects while ensuring adequate nutrient absorption. Increasing water intake to at least 2 litres daily and consuming 25-30g of dietary fibre from vegetables, wholegrains, and fruits can help maintain regular bowel movements.

Orlistat Dosage Options and Their Effects

Orlistat is available in two strengths in the UK: 60mg (available over the counter as Alli or Orlos) and 120mg (prescription-only as Xenical or generic orlistat). The 120mg prescription strength blocks approximately 25-30% of dietary fat, while the 60mg dose blocks around 25% [1]. Both strengths require you to take one capsule with each main meal containing fat, up to three times daily.

At Cured Pharmacy, we offer Orlos 60mg from £32.00 for patients who meet over-the-counter criteria, and prescription-strength Orlistat 120mg from £32.00 and branded Xenical from £32.00 following a free online consultation with our UK prescribers. The choice between strengths depends on your BMI, weight loss goals, and medical history, which our clinical team assesses during your consultation.

Patients taking the higher 120mg dose may experience more pronounced gastrointestinal effects initially, though these typically diminish within the first few weeks as you adjust your dietary fat intake [2]. Starting with the lower 60mg dose can help you gauge your tolerance before progressing to prescription strength if clinically appropriate.

Comparing Orlistat to Other Weight Loss Treatments

Unlike newer GLP-1 receptor agonist medications that work systemically to reduce appetite and slow gastric emptying, orlistat works locally in your digestive system without entering your bloodstream. This means orlistat doesn't carry the same risk of systemic side effects, making it suitable for patients who cannot tolerate injectable treatments or prefer an oral medication.

The trade-off is that orlistat requires strict adherence to a reduced-fat diet to be effective and to minimise gastrointestinal side effects. Clinical trials show that orlistat combined with a reduced-calorie diet produces an average additional weight loss of 2.9kg compared to diet alone over 12 months [3]. While this is more modest than results seen with newer treatments, orlistat remains a cost-effective option for many UK patients.

Orlistat is particularly suitable if you have a BMI of 28 or above with weight-related health conditions, or a BMI of 30 or above. All prescription medications at Cured Pharmacy require clinical assessment by a UK prescriber to ensure they're safe and appropriate for your individual circumstances.

Who Should Avoid Orlistat

Orlistat is not suitable if you have chronic malabsorption syndrome, cholestasis, or are pregnant or breastfeeding. It should be used with caution in patients taking anticoagulants like warfarin, as reduced vitamin K absorption may affect INR levels [4]. Your Cured Pharmacy prescriber will review your full medical history and current medications during your consultation to ensure orlistat is safe for you.

Preventing Constipation While Taking Orlistat

If you're concerned about constipation or experiencing it during orlistat treatment, several evidence-based strategies can help. First, ensure you're consuming adequate dietary fibre from vegetables, fruits, wholegrains, and legumes. Aim for at least 25-30g of fibre daily, spread across your meals rather than concentrated in one sitting.

Second, maintain proper hydration by drinking at least 2 litres of water throughout the day. Adequate fluid intake is essential for fibre to work effectively in promoting regular bowel movements. Third, don't reduce your fat intake too drastically — while you should limit fat to approximately 30% of daily calories, eliminating fat entirely can paradoxically worsen constipation and reduce absorption of fat-soluble vitamins.

Regular physical activity also supports healthy digestive function. Even moderate exercise like brisk walking for 30 minutes daily can improve bowel transit time and reduce constipation risk. If constipation persists despite these measures, consult your GP or pharmacist before using laxatives, as some may interact with orlistat or mask underlying issues requiring medical attention.

Ordering Orlistat Online from Cured Pharmacy

At Cured Pharmacy, we make accessing genuine UK-licensed orlistat straightforward and discreet. Our free online consultation takes under 3 minutes and is reviewed by UK-registered prescribers who assess whether orlistat is clinically appropriate for you. We offer transparent upfront pricing with no hidden fees — you'll see the exact cost before completing your consultation.

We stock multiple orlistat options to suit different budgets and preferences: Orlos 60mg from £32.00, generic Orlistat 120mg from £32.00, and branded Xenical 120mg from £32.00. All products are sourced from UK-licensed wholesalers and dispensed by our GPhC-registered pharmacy (registration number 9012511). We offer next day delivery across the UK in 100% discreet packaging.

Every prescription medication requires clinical approval from our UK prescribers. Once approved, your medication is dispensed the same day for next day delivery. Our clinical team is available on (+44) 116 4646009 if you have questions about orlistat's side effects, dosing, or suitability for your circumstances. We're committed to supporting your weight loss journey with expert pharmaceutical care and the lowest prices guaranteed in the UK.

What to Expect After Ordering

After your consultation is approved, you'll receive confirmation via email with tracking information for your delivery. Your medication arrives in plain, unmarked packaging with no reference to the contents or pharmacy name. If you experience any unexpected side effects or have concerns during treatment, contact our pharmacy team for advice. Regular follow-up with your GP or our prescribers helps ensure orlistat remains safe and effective for your weight management goals.

Scientific References

1. Guerciolini, R. (1997). Mode of action of orlistat. International Journal of Obesity, 21(Suppl 3), S12-S23. [accessed 13 August 2026]
2. Heck, A. M., Yanovski, J. A., & Calis, K. A. (2000). Orlistat, a new lipase inhibitor for the management of obesity. Pharmacotherapy, 20(3), 270-279. https://doi.org/10.1592/phco.20.4.270.34882 [accessed 13 August 2026]
3. Torgerson, J. S., Hauptman, J., Boldrin, M. N., & Sjöström, L. (2004). XENical in the prevention of diabetes in obese subjects (XENDOS) study: a randomized study of orlistat as an adjunct to lifestyle changes for the prevention of type 2 diabetes in obese patients. Diabetes Care, 27(1), 155-161. https://doi.org/10.2337/diacare.27.1.155 [accessed 13 August 2026]
4. Medicines and Healthcare products Regulatory Agency. (2009). Orlistat: Summary of Product Characteristics. UK Medicines Information. [accessed 13 August 2026]

Information on this page is for educational purposes only and is not medical advice. All prescription treatments require clinical assessment by a UK-registered prescriber. Always consult a qualified healthcare professional before starting any new medication. Orlistat should be used as part of a comprehensive weight management programme including dietary changes and increased physical activity. Individual results may vary.