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Can Orlistat Cause Constipation? Understanding Digestive Side Effects

Published on: June 03, 2026

Many patients ask can orlistat cause constipation UK pharmacies before starting treatment. In clinical practice, constipation is uncommon with orlistat — the medication typically causes the opposite effect due to its mechanism of blocking dietary fat absorption [1]. At Cured Pharmacy, our superintendent pharmacist Tarun Kumar regularly advises patients on managing orlistat's digestive effects, which differ significantly from typical weight loss medications.

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]. When you take orlistat with a meal containing fat, approximately 25-30% of that fat passes through your digestive system unabsorbed, eventually being eliminated in bowel movements [2].

Because unabsorbed fat moves through your bowel rather than being absorbed into your bloodstream, the most common digestive effects are oily stools, increased bowel frequency, and urgency — not constipation [1]. This is why orlistat's patient information leaflet emphasises the importance of maintaining a reduced-fat diet (below 30% of daily calories from fat) to minimise these effects.

Constipation is listed as an uncommon side effect (affecting fewer than 1 in 100 patients) in the UK Summary of Product Characteristics for orlistat [3]. When it does occur, it's typically related to dietary changes patients make when starting a weight loss programme, rather than the medication's direct pharmacological action.

Why Some Patients Experience Constipation on Orlistat

Whilst orlistat itself doesn't typically cause constipation, several factors related to starting a weight loss programme can lead to reduced bowel frequency. Understanding these helps you distinguish between medication effects and lifestyle changes.

The most common reason is dramatically reducing overall food intake. Many patients starting orlistat simultaneously adopt very low-calorie diets, which naturally produce less waste material for bowel movements [4]. Your digestive system needs sufficient volume to trigger regular bowel movements — eating too little can slow transit time.

Inadequate fibre intake is another frequent culprit. When patients focus on reducing dietary fat, they sometimes inadvertently reduce wholegrain carbohydrates, fruits, and vegetables that provide essential dietary fibre [4]. The NHS recommends 30g of fibre daily for healthy bowel function, yet the average UK adult consumes only 18g.

Dehydration and Physical Activity Changes

Insufficient fluid intake compounds the problem. Orlistat doesn't directly affect hydration, but patients focused on dietary changes sometimes forget to maintain adequate water consumption. Your colon absorbs water from waste material — without enough fluid intake, stools become harder and more difficult to pass [4].

Reduced physical activity can also contribute. Some patients decrease exercise when starting a calorie-restricted diet due to lower energy levels, yet regular movement stimulates intestinal motility and helps maintain regular bowel patterns.

Can Orlistat Cause Constipation UK: Clinical Evidence

The XENDOS study, a four-year randomised controlled trial involving 3,305 patients taking orlistat 120mg, provides robust long-term safety data [2]. Gastrointestinal events were the most commonly reported side effects, but these were predominantly fatty/oily stools (26.6% of patients), faecal urgency (22.1%), and oily spotting (23.9%) — symptoms consistent with orlistat's mechanism of action.

Constipation was reported by only 5.2% of orlistat patients compared to 4.5% of placebo patients in the XENDOS trial [2]. This minimal difference suggests constipation occurs at similar rates whether patients take orlistat or not, indicating it's more likely related to dietary changes than the medication itself.

A systematic review published in the British Journal of Clinical Pharmacology analysed safety data from multiple orlistat trials and confirmed that whilst gastrointestinal side effects are common, they're almost exclusively related to fat malabsorption rather than reduced bowel frequency [3]. The review noted that patients who adhered to recommended fat intake (less than 30% of calories) experienced significantly fewer digestive symptoms overall.

Managing Digestive Symptoms While Taking Orlistat

If you experience constipation whilst taking orlistat, focus first on dietary and lifestyle modifications rather than assuming the medication is responsible. Increase your fibre intake gradually by incorporating more vegetables, fruits, wholegrain bread, brown rice, and pulses into your reduced-fat meal plan.

Aim for at least 2 litres of water daily, spread throughout the day. Adequate hydration softens stools and supports healthy bowel transit time. Regular physical activity, even a 20-30 minute daily walk, stimulates intestinal motility and can help establish regular bowel patterns.

Maintain a consistent meal schedule. Eating at regular times helps regulate your digestive system's natural rhythm. If constipation persists despite these measures, speak with your prescriber before taking laxatives, as some may interact with fat-soluble vitamin absorption — already reduced by orlistat's mechanism of action.

When to Seek Medical Advice

Contact your GP or pharmacist if constipation lasts more than a week despite lifestyle modifications, if you experience severe abdominal pain, or if you notice blood in your stools. These symptoms warrant professional assessment regardless of whether you're taking orlistat.

Our clinical team at Cured Pharmacy can review your treatment during follow-up consultations. Superintendent pharmacist Tarun Kumar and our UK-registered prescribers assess whether orlistat remains appropriate for you based on your response to treatment and any side effects you experience.

Orlistat vs Xenical: Same Active Ingredient, Same Effects

Patients sometimes ask whether Xenical causes different digestive effects compared to generic orlistat. Both contain identical active ingredients at the same strengths — 120mg capsules for prescription products, or 60mg for over-the-counter Orlos (Alli) [1].

Xenical is the branded version manufactured by Cheplapharm, whilst generic orlistat is produced by various licensed pharmaceutical companies. Clinical trials showing orlistat's efficacy and side effect profile apply equally to Xenical, as they're bioequivalent medications approved by the MHRA to the same standards.

The digestive effects you experience depend on your dietary fat intake and individual physiology, not whether you choose branded Xenical (from £32.00) or generic orlistat (from £32.00). Both work through the same mechanism and carry the same side effect profile. Your choice typically comes down to personal preference and budget rather than clinical differences.

Buying Orlistat Online UK: What You Need to Know

Orlistat 120mg requires a prescription in the UK, whilst the 60mg strength (Orlos) is available after a pharmacy consultation. At Cured Pharmacy, all orlistat products undergo clinical assessment by our UK-registered prescribers to ensure suitability and safety.

Our online consultation takes under 3 minutes and covers your medical history, current medications, and weight loss goals. This assessment is mandatory — orlistat isn't suitable for everyone, particularly those with chronic malabsorption syndrome or cholestasis [3]. Your prescriber will also check you meet the BMI criteria (28 or above with risk factors, or 30 or above) required for NHS and private prescribing.

We stock genuine UK-licensed orlistat from £32.00, Xenical 120mg capsules from £32.00, and Orlos 60mg from £32.00. All products come with discreet packaging and include patient information leaflets covering proper use, dietary guidance, and what to expect. Your orlistat will be dispensed by our GPhC-registered pharmacy (9012511) and delivered directly to your address with tracked shipping.

Why Choose Cured Pharmacy for Orlistat

We guarantee the lowest prices in the UK with transparent upfront pricing — you'll see the exact cost before completing your consultation. Our clinical team provides ongoing support throughout your treatment, and you can contact us on (+44) 116 4646009 with any questions about managing side effects or optimising your weight loss programme.

Every orlistat product we dispense is sourced through UK-licensed wholesalers and stored in our GPhC-registered pharmacy premises. We never supply unlicensed or imported alternatives, ensuring you receive genuine medication that meets UK safety standards.

Scientific References

1. 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]
2. 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]
3. Padwal, R. S., & Majumdar, S. R. (2007). Drug treatments for obesity: orlistat, sibutramine, and rimonabant. The Lancet, 369(9555), 71-77. https://doi.org/10.1016/S0140-6736(07)60033-6 [accessed 13 August 2026]
4. Slavin, J. L. (2005). Dietary fiber and body weight. Nutrition, 21(3), 411-418. https://doi.org/10.1016/j.nut.2004.08.018 [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.