Orlistat Side Effects: Complete Safety Guide
When Do Side Effects Start? How Long Do They Last? Timeline, Contraindications & Food Interactions
Key Takeaways: Orlistat Side Effects
- Most common: Oily spotting, urgent bowel movements, fatty stools, flatulence with discharge – these affect >1 in 10 users.
- Start: Usually within 24-48 hours, especially after high‑fat meals.
- Last: As long as you take orlistat with high‑fat foods; they diminish with diet adjustment.
- Contraindications: Malabsorption, cholestasis, pregnancy, breastfeeding, allergy.
- Food interaction: High‑fat meals worsen GI side effects; take with meals containing fat.
- Serious but rare: Liver injury, pancreatitis, kidney stones – seek help if symptoms occur.
Orlistat is generally well tolerated, but like all medicines, it can cause side effects. Most are related to its action in the gut and can be managed by following a low‑fat diet.
Important Medical Advice
Seek immediate medical help if you experience: severe abdominal pain, persistent vomiting, yellowing of skin or eyes (jaundice), dark urine, severe allergic reaction (rash, itching, swelling, difficulty breathing), or symptoms of pancreatitis (severe upper stomach pain spreading to back).
Orlistat Side Effects: Overview & Common Side Effects
Orlistat works locally in your gut, so most side effects are related to the digestive system. They are most noticeable when you eat too much fat. Based on the Patient Information Leaflet (PIL) PL 00289/1680 (July 2022), side effects are categorised by frequency:
Very common (affect more than 1 in 10 people)
- Headache
- Upper respiratory tract infection
- Abdominal pain or discomfort
- Oily spotting from the rectum
- Urgent or increased need to open bowels
- Flatulence (wind) with discharge
- Oily or fatty stools
- Liquid stools
- Low blood sugar (in people with type 2 diabetes)
- Flu (influenza) infection
Common (affect up to 1 in 10 people)
- Lower respiratory tract infection
- Rectal pain/discomfort
- Soft stools
- Incontinence (stools)
- Bloating (in people with type 2 diabetes)
- Tooth or gum disorder
- Urinary tract infection
- Irregular menstrual cycle
- Tiredness
- Anxiety
Rare / frequency not known
- Allergic reactions (itching, rash, wheals, difficulty breathing, nausea, vomiting)
- Rectal bleeding
- Diverticulitis
- Gallstones
- Hepatitis (inflammation of the liver) – yellow skin/eyes, dark urine
- Pancreatitis
- Oxalate nephropathy (kidney stones) – especially in those with chronic kidney disease
- Increased liver enzymes
- Skin blistering
🗒️ Note: The gastrointestinal side effects are a direct result of undigested fat passing through your system. They are not harmful but can be unpleasant. Sticking to a low‑fat diet reduces them significantly.
When Do Orlistat Side Effects Start?
Orlistat begins working within hours of your first dose, so side effects can appear quickly – often after the first meal that contains fat.
- First 24-48 hours: Most people notice changes in bowel habits within the first day or two, especially if their diet is not yet adjusted.
- After high‑fat meals: Symptoms like oily spotting or urgent stools can occur within 1-3 hours of eating a fatty meal.
- Peak onset: Side effects are most pronounced during the first few weeks of treatment as your body adjusts and you learn to avoid high‑fat foods.
If you suddenly increase fat intake later in treatment, side effects can return quickly.
How Long Do Orlistat Side Effects Last? (Timeline)
The duration depends on your diet and whether you continue the medication.
| Situation | How long side effects last |
|---|---|
| After a single high‑fat meal | Usually a few hours to 24 hours – until the undigested fat passes. |
| During continuous treatment with proper low‑fat diet | Mild effects may occur initially, but often subside within 1-2 weeks as you adapt. |
| If you continue high‑fat diet | Side effects persist as long as you eat too much fat. They will stop 2-3 days after you stop orlistat. |
| After stopping orlistat | Normal digestion returns within 48-72 hours, and side effects disappear. |
Serious side effects like liver injury or pancreatitis are rare and require immediate medical attention; their duration varies based on treatment.
Orlistat Contraindications: Who Should Not Take It
Do not use orlistat if any of the following apply:
- Allergy: To orlistat or any ingredient (microcrystalline cellulose, sodium starch glycolate, silica, sodium lauril sulfate, gelatin, titanium dioxide, indigo carmine).
- Chronic malabsorption syndrome: Conditions where you cannot absorb nutrients properly (e.g., Crohn's disease, cystic fibrosis).
- Cholestasis: A liver disorder where bile flow is reduced.
- Breastfeeding: Orlistat may pass into breast milk and could harm the baby.
- Pregnancy: Not recommended – weight loss offers no benefit and safety is unknown.
- Children and adolescents under 18: Not intended for use.
- Concurrent ciclosporin: Co‑administration not recommended unless closely monitored.
- Concurrent acarbose: Not recommended (diabetes medicine).
If you have chronic kidney disease, use with caution due to risk of kidney stones. Always discuss your full medical history with a doctor.
Food Interactions with Orlistat
Orlistat works by blocking fat absorption, so its effects – and side effects – are directly influenced by what you eat.
Foods that increase side effects
- High‑fat meals (more than 30% of calories from fat) – trigger oily spotting, diarrhoea, urgent stools.
- Fatty snacks between meals – can cause unexpected GI symptoms because you haven't taken a dose.
Foods that help minimise side effects
- Low‑fat, well‑balanced meals with moderate fat (approx. 15g per meal).
- Plenty of fruits, vegetables, and fibre – they help regulate bowel movements.
- Spreading fat intake evenly across three meals.
Fat‑soluble vitamins
Orlistat reduces absorption of vitamins A, D, E, and K. Take a multivitamin supplement at bedtime (at least 2 hours after your last orlistat dose) to compensate.
🗒️ Practical tip: Keep a food diary for the first few weeks to identify which fat levels suit you. Aim for meals with <15g fat.
Orlistat Side Effects FAQs
What are the most common side effects of orlistat?
Very common side effects (affecting more than 1 in 10) include oily spotting, urgent need to open bowels, fatty/oily stools, flatulence with discharge, and abdominal pain. These are usually mild and improve with a low-fat diet.
When do orlistat side effects start?
Side effects typically begin within the first 24-48 hours of starting treatment, often after the first few meals containing fat. They are most noticeable when you eat a high-fat meal.
How long do orlistat side effects last?
Most side effects last only as long as the drug is in your system and usually subside within a few days as you learn to avoid high-fat foods. If you continue a high-fat diet, they may persist. After stopping orlistat, side effects disappear within 2-3 days.
Can I drink alcohol while taking orlistat?
Alcohol itself does not interact with orlistat, but alcoholic drinks are often high in calories and can hinder weight loss. Moderate consumption is acceptable but be mindful of calorie intake.
Who should not take orlistat?
Orlistat is contraindicated in people with chronic malabsorption syndrome, cholestasis, breastfeeding women, and those allergic to orlistat. It is not recommended during pregnancy or with ciclosporin/acarbose.
Need a Prescription for Orlistat?
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Medical Content Manager
Nabeel is a co-founder, and medical content manager of Chemist Doctor. He works closely with our medical team to ensure the information is accurate and up-to-date.
Medical Doctor
Dr. Feroz is a GMC-registered doctor and a medical reviewer at Chemist Doctor. He oversees acute condition and urgent care guidance.
Medical Director
Usman is a co-founder, and medical director of Chemist Doctor. He leads the organisation's strategic vision, bridging clinical and operational priorities.
Review Date: 23 February 2026
Next Review: 23 August 2026
Published on: 23 February 2026
Last Updated: 23 February 2026