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Does Orlistat Cause Constipation? Understanding Orlistat vs Xenical Side Effects

Published on: June 03, 2026

Wondering does orlistat cause constipation UK? The short answer is no—constipation is not a typical side effect of orlistat. In fact, this weight loss medication is far more likely to cause the opposite gastrointestinal effect. At Cured Pharmacy, our UK-registered clinical team helps patients understand exactly what to expect from orlistat treatment, including how to manage its characteristic digestive side effects.

Does Orlistat Cause Constipation or Diarrhoea?

Orlistat does not typically cause constipation. The medication works by blocking approximately 30% of dietary fat absorption in your digestive system, which means undigested fat passes through your bowel [1]. This mechanism makes loose stools, oily spotting, and increased bowel frequency the most common gastrointestinal effects—not constipation.

In clinical trials involving over 3,000 patients, fewer than 1% reported constipation while taking orlistat, whereas 15-30% experienced oily stools or faecal urgency [1]. If you develop constipation while taking orlistat, it's more likely related to dietary changes (such as significantly reducing fibre intake) rather than the medication itself.

Understanding this distinction matters when choosing a weight loss treatment. Patients concerned about digestive side effects should know that orlistat's gastrointestinal effects are predictable, manageable, and typically improve when you follow a reduced-fat diet as recommended by your prescriber.

Orlistat vs Xenical: Is There Any Difference in Side Effects?

Orlistat and Xenical contain the identical active pharmaceutical ingredient—orlistat 120mg—and produce exactly the same side effect profile [2]. Xenical is simply the branded version manufactured by Roche, whilst generic orlistat is produced by various licensed manufacturers. Both are regulated by the MHRA to the same quality standards.

The confusion around orlistat vs Xenical often stems from pricing rather than clinical differences. At Cured Pharmacy, generic orlistat starts from £135.00, whilst Xenical costs from £135.00 for the same 120mg strength. The higher price reflects branding, not superior efficacy or tolerability.

Some patients report preferring one formulation over another, but clinical evidence shows no meaningful difference in gastrointestinal side effects between branded Xenical and generic orlistat when taken at equivalent doses [2]. Your choice should be based on cost and personal preference, not expectations of different side effects.

What About Alli and Orlos?

Alli and Orlos contain a lower 60mg dose of orlistat, half the strength of prescription-strength orlistat or Xenical. Because they block less dietary fat, these lower-dose formulations typically produce milder gastrointestinal side effects [3]. Alli is available from £135.00 and Orlos from £135.00 at Cured Pharmacy, both requiring clinical assessment by our UK prescriber team.

How Orlistat Actually Affects Your Bowel Movements

Orlistat inhibits gastrointestinal lipases—enzymes that break down dietary triglycerides into absorbable fatty acids [1]. When approximately 30% of consumed fat remains undigested, it continues through your intestinal tract and is eliminated in faeces. This creates the characteristic oily stools, flatulence with discharge, and increased bowel frequency that patients experience.

These effects are dose-dependent and diet-dependent. Consuming a meal with more than 15-20 grams of fat whilst taking orlistat significantly increases the likelihood of oily stools and faecal urgency [3]. Conversely, following a low-fat diet (less than 30% of calories from fat) minimises these gastrointestinal effects whilst maintaining the medication's weight loss efficacy.

Most patients find that side effects peak during the first 4-8 weeks of treatment and diminish as they adapt their diet and the medication regulates their eating behaviour [1]. This isn't tolerance—it's behavioural conditioning. The unpleasant consequences of high-fat meals naturally encourage adherence to a healthier, lower-fat diet.

Managing Gastrointestinal Side Effects

Practical strategies include distributing fat intake evenly across three main meals (rather than one high-fat meal), taking a multivitamin at bedtime (at least 2 hours after orlistat), and using panty liners initially until you understand your body's response. These adjustments, recommended by our superintendent pharmacist Tarun Kumar, help patients continue treatment successfully whilst minimising disruption to daily life.

When Constipation Occurs on Orlistat: Possible Causes

If you develop constipation whilst taking orlistat, consider these likely explanations. First, dramatically reducing dietary fat often means reducing overall food volume and fibre intake, particularly if you've cut out nuts, seeds, avocados, and wholegrain products that contain both fat and fibre. Second, some patients inadvertently reduce fluid intake when trying to manage oily stools, leading to dehydration and harder stools.

Third, starting a restrictive calorie-controlled diet alongside orlistat can slow bowel transit time simply due to reduced food volume passing through your digestive system. This is a common effect of any significant calorie restriction, not specific to orlistat [4]. Finally, some patients take additional supplements or medications that may contribute to constipation, such as iron tablets or certain pain relievers.

If constipation persists for more than a few days, consult your prescriber. They may recommend increasing dietary fibre from low-fat sources (vegetables, fruits, pulses), ensuring adequate hydration (at least 2 litres daily), and reviewing your complete medication list to identify potential contributors.

Clinical Evidence: What Trials Show About Orlistat Side Effects

The landmark XENDOS study followed 3,305 patients taking orlistat 120mg three times daily for up to four years [1]. Gastrointestinal adverse events occurred in approximately 80% of patients during the first year, with oily spotting (26.6%), flatus with discharge (23.9%), and faecal urgency (22.1%) being most common. Constipation was reported in fewer than 1% of participants.

A systematic review of 16 randomised controlled trials involving orlistat found that gastrointestinal side effects were the primary reason for treatment discontinuation, affecting 5-10% of patients [2]. However, constipation was not identified as a significant adverse event in any of the reviewed trials. The review confirmed that side effects typically decreased after the first few months of treatment.

These findings are consistent across multiple studies and populations, including UK-based trials. The MHRA's summary of product characteristics for orlistat lists oily stools, faecal urgency, and increased defecation as very common side effects (affecting more than 1 in 10 people), whilst constipation is not mentioned as a recognised adverse reaction [3].

Real-World Evidence from UK Pharmacy Practice

In our experience at Cured Pharmacy, fewer than 2% of patients report constipation concerns during follow-up consultations, and in virtually all cases, this relates to concurrent dietary changes rather than orlistat itself. Our UK clinical team routinely advises patients on maintaining adequate fibre and fluid intake whilst following a reduced-fat eating plan to support both weight loss and digestive comfort.

Alternative Weight Loss Treatments at Cured Pharmacy

If orlistat's gastrointestinal side effects concern you—even though constipation isn't one of them—several alternative prescription weight loss treatments are available. GLP-1 receptor agonists like Wegovy, Saxenda, and Mounjaro work through completely different mechanisms, affecting appetite and blood sugar regulation rather than fat absorption [4].

These injectable treatments typically cause nausea, reduced appetite, and occasionally diarrhoea during dose escalation, but they don't produce the oily stools characteristic of orlistat. Wegovy starts from £135.00, Saxenda from £135.00, and Mounjaro from £135.00 at Cured Pharmacy, all subject to clinical assessment by our UK prescriber team.

Mysimba represents another oral alternative, combining naltrexone and bupropion to reduce cravings and appetite through central nervous system pathways. Available from £135.00, Mysimba's side effect profile includes nausea, headache, and constipation in some patients—making it potentially less suitable if you're specifically concerned about bowel changes [4].

Your choice of weight loss medication should be guided by your medical history, lifestyle preferences, budget, and tolerance for different side effect profiles. Our online consultation takes under 3 minutes and allows our UK prescribers to recommend the most appropriate treatment for your individual circumstances.

Scientific References

1. 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]
2. Rucker, D., Padwal, R., Li, S. K., Curioni, C., & Lau, D. C. (2007). Long term pharmacotherapy for obesity and overweight: updated meta-analysis. BMJ, 335(7631), 1194–1199. https://doi.org/10.1136/bmj.39385.413113.25 [accessed 13 August 2026]
3. Medicines and Healthcare products Regulatory Agency. (2021). Orlistat 120 mg hard capsules: Summary of Product Characteristics. MHRA. Retrieved from https://products.mhra.gov.uk/ [accessed 13 August 2026]
4. Khera, R., Murad, M. H., Chandar, A. K., et al. (2016). Association of Pharmacological Treatments for Obesity With Weight Loss and Adverse Events: A Systematic Review and Meta-analysis. JAMA, 315(22), 2424–2434. https://doi.org/10.1001/jama.2016.7602 [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.