What are the most common orlistat side effects?

In trials the most commonly reported were oily or fatty stools and oily spotting (about 27% of participants each), faecal urgency (22%), abdominal pain or discomfort (14%) and faecal incontinence (8%). The Xenical label reports similar first-year rates against placebo — oily spotting 26.6% versus 1.3%, and faecal urgency 22.1% versus 6.7%.

Why do the side effects happen at all?

Orlistat blocks pancreatic lipase in the small intestine, which stops roughly 30% of the fat in your food from being broken down and absorbed. That undigested fat passes through the gut. The gut effects are the mechanism working, and they scale with how much fat is in your meals.

Do I need to take a vitamin supplement with orlistat?

Both labels recommend it. Because orlistat blocks fat absorption, it also reduces absorption of the vitamins that dissolve in fat. The Xenical label says to take a multivitamin containing fat-soluble vitamins at least two hours away from a dose; the Alli label says to take a multivitamin once a day, at bedtime.

Does orlistat interact with other medicines?

The Xenical label singles out two: ciclosporin should be taken at least three hours before or after orlistat, and levothyroxine at least four hours apart. The Alli label also tells people to check with a doctor or pharmacist if they take anticoagulants, amiodarone, diabetes medicines, thyroid medicines, seizure medicines or antiretrovirals.

Is Alli safer than prescription Xenical?

Alli is the same drug at half the strength — 60 mg over the counter versus 120 mg on prescription. The over-the-counter label describes the same bowel changes and the same multivitamin advice, and says not to use it if you take ciclosporin, have been diagnosed with problems absorbing food, or are not overweight.

Xenical and Alli side effects

What the FDA label warns about

These come from the Xenical prescribing information and the Alli over-the-counter Drug Facts label, both published on DailyMed.

Vitamin absorption: the warning that changes your routine

Because orlistat blocks fat absorption, it also reduces absorption of the vitamins that dissolve in fat. The Xenical label says people taking it should take a multivitamin containing fat-soluble vitamins to ensure adequate nutrition, and that the supplement should be taken at least two hours before or after a dose — at bedtime, for example. The Alli label gives the same advice more simply: take a multivitamin once a day, at bedtime.

Timing matters for some other medicines too. The Xenical label says ciclosporin should be taken at least three hours before or after orlistat, and levothyroxine at least four hours apart.

Other warnings and precautions on the label:

Drug interactions and decreased vitamin absorption
Liver injury — there have been rare reports after marketing of severe liver injury, including liver failure
Oxalate kidney stones and oxalate nephropathy with kidney failure
Gallstones

The labels say orlistat must not be used if you:

Are pregnant
Have chronic malabsorption syndrome
Have cholestasis
Are allergic to orlistat or anything else in the capsule
Take ciclosporin, or have been diagnosed with problems absorbing food (from the Alli label)
Are not overweight (from the Alli label)

The label numbers, against placebo

The Xenical label reports these first-year gastrointestinal event rates against placebo.

Adverse reaction rates reported on the Orlistat FDA label, against placebo
Reaction	XENICAL, year 1	Placebo
Oily spotting	26.6%	1.3%
Flatus with discharge	23.9%	1.4%
Faecal urgency	22.1%	6.7%
Fatty or oily stool	20.0%	2.9%
Oily evacuation	11.9%	0.8%
Increased defecation	10.8%	4.1%
Faecal incontinence	7.7%	0.9%

Rare but serious — when to call your doctor

Call your doctor immediately if you have:

Yellowing of the skin or eyes, dark urine, itching, or pain over your liver (possible liver injury)
Severe pain in your side or back, or blood in your urine (possible kidney stone)
Pain in the upper right of your abdomen, particularly after eating (possible gallstones)
Passing much less urine than usual (possible kidney problems)
Severe or persistent abdominal pain
Rectal bleeding

What actually helps

Strategies that work best, based on trial and community experience.

Multivitamin at bedtime

Both labels recommend a daily multivitamin containing fat-soluble vitamins, taken at least two hours away from a dose — bedtime is the timing the labels themselves suggest.

Fat in the meal drives the effect

The gut effects scale with how much fat you eat. Both labels pair the drug with a reduced-calorie, lower-fat diet, and lower-fat meals make the effects far less noticeable.

Space out other medicines

The Xenical label says ciclosporin should be taken at least three hours apart from orlistat and levothyroxine at least four hours apart. Check anything else with your pharmacist.

Plan for the first weeks

Dark clothing and liners are the practical tips users share most often, especially early on while you work out how your meals affect you.

When to push through vs. talk to your doctor

You can push through if...

Oily stools or spotting after a higher-fat meal
Wind, urgency or more frequent bowel movements early on
Effects that get more predictable as you learn how your meals affect you
Mild abdominal discomfort

Talk to your doctor if...

Yellowing skin or eyes, dark urine, or itching
Severe pain in your side, back or abdomen
Rectal bleeding
Faecal incontinence that is affecting daily life
Any symptom you are unsure about

Common questions

Written and maintained by one person · Built with agentic AI tools · Every number checked against the cited trial · Not medical advice

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Keep reading

Full Xenical and Alli overview →Xenical and Alli trial results →What to expect week by week →Xenical and Alli cost →All orlistat clinical evidence →Questions to ask your doctor →

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Built with agentic AI tools and not a substitute for medical advice

Last reviewed: September 2026

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