Once you start taking Orlistat, your body will stop absorbing some fats. This is why oily stools, flatulence, diarrhoea, and the urge to use the restroom are common side effects. While such problems can be extremely irritating, they usually indicate the drug’s effectiveness.
Digestive side effects are common with Orlistat due to high-fat intake. Knowing what to expect can help you manage these symptoms easily with simple dietary changes. This guide covers the most common side effects, practical ways to reduce them, and severe symptoms that require immediate medical attention.
Key Takeaways
- Orlistat supports weight loss by blocking the absorption of around one-third of the fat you eat.
- Most Orlistat side effects are digestive, including oily stools, bloating, wind, diarrhoea, and an urgent need to use the toilet.
- Following a low-fat diet and limiting fat to around 15-20g per meal can help reduce side effects.
- Take Orlistat with meals that contain fat, and skip the dose if you miss a meal or eat a fat-free meal.
- Serious side effects are uncommon; however, symptoms such as severe stomach pain, dark-colored urine, yellowing of the skin or eyes, and difficulty breathing require immediate attention from your physician.
- Orlistat may interact with some drugs and fat-soluble vitamins. Therefore, always adhere to your physician's instructions.
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What Causes Orlistat Side Effects
Orlistat works differently from other weight-loss medications, as it acts on the gastrointestinal tract rather than the brain. The drug inhibits enzymes that break down fats in your digestive system. Consequently, nearly one-third of the fats you consume leave your body undigested.
The undigested fat leaves the body through bowel movements. This is why digestive symptoms are the most common side effects of Orlistat. If you eat a meal that contains more fat, you'll notice symptoms such as oily or fatty stools, bloating, wind, or an urgent bowel movement.
The National Health Service (NHS) Inform states that Orlistat begins to affect fat digestion within 1 to 2 days. Treatment is usually reviewed after three months, and you can continue if you’ve lost at least 5% of your starting body weight during that period.
Start your weight-loss journey with orlistat (generic Xenical). Take 120mg Orlistat capsules three times a day with meals containing fat. Ensure you take the dose immediately before, during, or up to one hour after a main meal.
Importance of Diet When Taking Orlistat Medication
The amount of fat in your diet determines both the effectiveness of Orlistat and its side effects. Health care specialists always recommend that no more than 30% of daily calories should come from fat.
To be more specific, the NHS recommends aiming for about 15-20 grams of fat per main meal. This means you should eat a healthy, low-fat, balanced diet all day.
Healthy diets consist mainly of lean cuts of meat, vegetables, skimmed milk, low-fat butter, and grilled dishes. Stay away from deep-fried dishes, high-fat sauces, cakes, ready-made meals, and too much full-fat cheese.
Common Side Effects of Orlistat
Gastrointestinal issues are the most frequently observed side effects of Orlistat. The NHS suggests that such side effects can occur less often if you follow a low-fat diet.
Digestive Side Effects
| Side Effect | Why It Happens | What May Help |
| Fatty or oily poo | Undigested fat, oily poo needing urgent toilet visits, or bloating. | A reduced-calorie diet (with no more than 30% of your calories from fat) and increased movement. |
| Oily discharge | Fat leaks during bowel movements. | Spread fat intake evenly. |
| Urgent need to poo | The bowel reacts to undigested fat. | Avoid foods high in fat. |
| More frequent bowel movements | Fat moves through the digestive system faster. | Stick to regular lower-fat meals. |
| Wind and bloating | Digestive changes from fat malabsorption. | Limit fried foods and creamy sauces. |
| Stomach pain or abdominal pain | Digestive irritation linked to fat intake. | Reduce portion sizes and fat intake. |
| Diarrhoea | Excess fat in the bowel | Stay hydrated and review your diet |
Some people also experience less common side effects. These may include:
- Irregularity of menstrual cycle
- Incontinence (stools)
- Rectal pain/discomfort
- Soft stools
- Tooth/gum disorder
- Tiredness
Contact your GP if you notice any of these symptoms.
How Long Do Side Effects Last?
Many people experience the worst side effects during the first few weeks after starting Orlistat. However, gastrointestinal symptoms often improve once you learn which foods trigger problems. They happen during the first three months of treatment. A large number last less than one week, while most settle within four weeks.
Don't ignore persistent severe diarrhoea, severe abdominal pain, or symptoms that interfere with everyday life. Contact your prescriber if symptoms continue despite following dietary advice carefully.
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How to Reduce Side Effects While Taking Orlistat
Follow a Lower-Fat Diet
The most effective way to reduce Orlistat side effects is to lower the amount of fat you eat. Try to spread your daily fat evenly rather than saving it for one meal. The following meals can feel easier on your digestive system:
- A breakfast with skimmed milk, low-fat spread, and wholegrain cereal.
- A lunch with salad dressings used sparingly.
- A dinner built around lean protein and vegetables.
Avoid foods high in fat, as they're the biggest trigger for oily stools, oily discharge, and the urge to use the toilet. These include:
- Fried foods
- Pastries
- Creamy sauces
- Fast food
- Full-fat cheese
- Processed snacks
- Large portions of fatty meats
Take Orlistat Correctly
Taking Orlistat exactly the same way each day can help reduce problems. The standard prescription dose is one 120mg capsule up to three times daily with meals containing fat. Take it immediately before, during, or up to one hour after eating.
If you miss a meal or eat a fat-free meal, simply skip the dose and continue as normal with your next meal containing fat. Taking Orlistat without dietary fat to block will not improve your results and may increase unnecessary digestive discomfort. Stick to a regular dosing schedule after meals to maintain consistent treatment results.
Think About Vitamins
Because Orlistat blocks fat absorption, it can also reduce the absorption of fat-soluble vitamins A, D, E, and K. NICE CKS guidance notes that this can affect nutritional intake over time. Doctors recommend taking a multivitamin supplement at least two hours after taking Orlistat, often before bed. This can help maintain healthy levels of beta carotene and other nutrients.
Always follow the patient information leaflet and your healthcare professional’s advice before starting supplements.
Serious Side Effects and When to Stop Taking Orlistat
Severe side effects are rare, but it’s important to know when symptoms need urgent medical advice.
Symptoms that Need Immediate Medical Attention
According to the NHS, you should stop taking Orlistat and seek medical attention if you notice:
- Severe abdominal pain
- Yellow skin or eyes
- Dark urine
- Pale stools
- Breathing problems
- Rash, hives, or signs of an allergic reaction
- Severe diarrhoea causing dehydration
- Unusual tiredness
- Signs of kidney problems or kidney stones
These symptoms may indicate liver problems, allergic reactions, or complications that require medical review. If you have certain medical conditions, such as chronic malabsorption syndrome or anorexia nervosa, don't take Orlistat unless specifically advised by a specialist.
Orlistat and Other Medications
The NHS Inform states that Orlistat can affect how other medicines work. This includes blood thinners, epilepsy medicines, thyroid medication used for an underactive thyroid, and medicines such as ciclosporin. Talk to your doctor if you're taking medications that suppress the immune system. If you're taking cyclosporine (Neoral, Sandimmune), take it 3 hours after orlistat.
If you experience severe diarrhoea, seek advice from a healthcare professional. Always disclose all prescription medicines, over-the-counter products, and supplements during your consultation.
Can Orlistat Fit into Your Weight-Loss Plan?
Who's Suitable
According to NICE, prescribed orlistat is suitable for overweight adults, those with obesity or additional health risks, such as high blood pressure or type 2 diabetes. It works best if you combine treatment with a low-fat diet, regular exercise program, and realistic long-term lifestyle changes.
Orlistat was the first prescription-only medicine licensed in the UK to help you lose weight. It’s a safe alternative to buying unregulated treatments online. These capsules are also a good choice if you can’t or don’t want to use weight loss injections such as Mounjaro, Wegovy, or Saxenda.
Curely Integration
At Curely, we provide UK-licensed weight loss treatment through a quick online consultation reviewed by prescribing professionals. The process is straightforward:
- Complete a short health questionnaire
- Select the most suitable medication
- Receive discreet delivery to your home or workplace
There’s no need for GP appointments or pharmacy visits, and all orders over £20 include free delivery. Begin your consultation now.
Frequently Asked Questions
What Should I Do After a Missed Dose?
If you miss a dose of Orlistat, don't take an extra capsule to make up for it. Simply take your next dose after a main meal that contains fat. If you miss a meal or eat a no-fat meal, skip the dose altogether. Taking Orlistat without dietary fat to block will not improve your results and increases the risk of side effects.
How Much Fat Should I Eat Per Meal?
The NHS Inform suggests limiting fat intake to about 15-20g per main meal, approximately 30% of your total daily calorie intake. By following a low-fat diet, you can minimise symptoms like fatty stools, flatulence, and urgency to visit the loo.
Can Orlistat Interfere With The Contraceptive Pill?
Orlistat doesn't affect the contraceptive or combined pill directly. However, if you develop severe diarrhoea while taking Orlistat, your body may not absorb oral contraceptives properly. Consult your GP and consider using additional contraception for protection.
When Should I Stop Taking Orlistat?
If you experience symptoms such as severe stomach pain, yellowing of the skin or eyes, dark urine, breathing difficulties, or signs of an allergic reaction, stop taking orlistat and consult your prescriber. Although rare, serious side effects can include liver problems. Also, review your treatment after three months to check if you've lost at least 5% of your starting body weight.
Is Orlistat Safe Long Term?
Orlistat has been licensed for weight loss in the UK for many years and is safe for many adults when prescribed by a healthcare professional. However, it isn't suitable for everyone. Regular reviews are important, particularly because Orlistat can affect the absorption of certain vitamins and interact with some medications.
What's the Difference Between Orlistat and Other Weight Loss Medications?
Orlistat works by blocking the absorption of around one-third of the fat you eat, which then passes through the body as waste. Other weight-loss medications, such as Mounjaro, work differently by regulating appetite and reducing hunger. The most suitable treatment depends on your medical history, weight loss goals, and lifestyle.
Conclusion
Most Orlistat side effects are digestive and often improve when you follow a low-fat diet and take the medication correctly. Controlling your fat intake can help reduce symptoms such as oily stools, bloating, and the urge to use the toilet.
Although serious side effects are rare, seek medical advice straight away if you develop symptoms such as severe stomach pain, dark urine, yellowing of the skin or eyes, or breathing difficulties.
Thinking about Orlistat for weight loss? Complete a quick online consultation with our doctors to find out if Orlistat or another clinically proven weight-loss treatment is suitable for you.
References
- National Health Service. (2026, May 10). Orlistat: A medicine to treat obesity. NHS.
- National Health Service Inform. (2026). Orlistat. NHS Inform.
- National Institute for Health and Care Excellence. (2025). Prescribing orlistat, liraglutide, and semaglutide. NICE.
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