If you’re exploring weight loss treatment options, orlistat is probably on your radar.
It works by reducing the amount of fat your gut absorbs from food. About one-third of the fat you eat is prevented from being digested, and is then passed out in your poo. And roughly one in five people taking it will lose at least 10% of their starting weight after a year.
But, like all medicines, orlistat comes with side effects. While these are usually nothing to worry about, it’s important to know what sort of symptoms you might expect while using it—and how you can deal with them. We’ve explained it all for you below.
Common side effects of orlistat
Because of how orlistat works, its side effects are most commonly related to the gut . The standard dose of orlistat is 120mg and it’s taken three times per day around mealtimes.
Side effects are usually mild and clear up on their own. They tend to happen when you first start on orlistat and after eating meals that are high in fat.
So, what symptoms should you look out for? Read on—these are the most common side effects reported, affecting over one in ten people using orlistat.
Stomach pain or discomfort
Orlistat’s actions on the gut—like having undigested fat pass through your system—can sometimes cause discomfort or stomach pain. But if your pain is severe or doesn’t go away, check in with your doctor.
Increased need to pass stools
And thanks to these effects on the gut, you might also need to go to the toilet more often to pass stools. In clinical trials , just over one in five people using orlistat experienced this.
Oily discharge, fatty or liquid stools
If more fat from your diet comes out in your stool, it follows that those stools might be more oily or runny in consistency. You might also have some oily discharge.
Flatulence
Flatulence (farting) happens when you have excess gas in your digestive tract, which normally forms as you digest food. Orlistat’s effect on the gut can make you gassy and pass more wind.
Headache
Almost 31% of people taking orlistat reported this symptom in clinical research .
Less common side effects
Here we have symptoms that might affect between 1-10 people in 100 using orlistat.
Heads up: this is not a complete list of all the side effects that have been reported. We’ve just included some of the most common to give you an idea of what to expect. Do check in with your provider if you’re not sure whether orlistat is a fit for you.
Soft stools
We mentioned earlier how orlistat can affect the consistency of your stools—well, this can also cause them to become soft.
Incontinence
Just under 8% of people find they have trouble controlling when they’re able to pass stools when using orlistat.
Nausea
Orlistat can upset your stomach and cause nausea. Around 8% of those using orlistat will experience this during their first year of taking it.
Tiredness
According to clinical research, around 7% of people using orlistat will feel fatigued.
Nutritional deficiencies
For some people, orlistat can affect how well they absorb fat-soluble vitamins, including vitamins A, D, E, K, and beta-carotene. If your levels of those vitamins are too low, it can have knock-on symptoms like fatigue, weakness, and skin problems. To get around this, it’s recommended that you take multivitamin supplements at least two hours before or after you use orlistat.
Rare but serious side effects
As we’ve mentioned, most orlistat side effects go away on their own. In clinical trials , around half of all gut symptoms lasted less than a week. In most cases, they cleared within a month.
Though some are more serious and last longer. If you have any of the below symptoms, seek help from your doctor right away.
FYI: for some of these symptoms, we don’t have enough data to estimate how many people they could affect.
Rectal bleeding
See a doctor if your stools are black or dark red, or if you have bloody diarrhoea. Get urgent help if you’re bleeding non-stop or there’s a lot of blood.
Allergic reactions
You might have swelling of the lips, face or throat, and/or have trouble breathing. Some people can also get itchy rashes and might feel unwell.
Long-term side effects of orlistat
It’s rare, but there can be long-term side effects from orlistat. The below are estimated to affect less than one in 10,000 people.
Hepatitis (inflammation of the liver)
Some warning signs of liver problems to look out for are yellowing skin and eyes, itching, dark-coloured urine, stomach pain and liver tenderness.
Gallstones
This is when small, hard masses form in your gallbladder. They’re usually made of cholesterol , bile salts and bilirubin, and can trigger sudden, intense pain in your stomach.
Managing and reducing side effects
Remember earlier, when we explained that eating higher-fat meals is likely to bring on side effects? So, to prevent that from happening, the advice is to choose foods that are low in fat.
For example, instead of eating sausages and fried meats, you could go for leaner cuts of beef, chicken, or turkey.
If you eat dairy, opt for lower-fat options (skimmed milk, cottage cheese) and try to avoid high-fat processed foods altogether (like pizza, burgers, and chips). Steer clear of foods that are naturally high in fat, too, like avocados and nuts.
As a general rule: if you look on a nutrition label, the total fat content should be less than 5g per 100g.
When to consult a doctor
Any of the symptoms mentioned above that are linked to serious side effects warrant a visit to the doctor.
To refresh your memory, those include: severe pain in your stomach or underneath your ribs on the right-hand side, signs of an allergic reaction, rectal bleeding, yellowing of the skin or eyes and other warning signs of liver problems.
You should also see your doctor if any of your symptoms are bothering you, getting worse, or don’t go away. Or if the medicine isn’t having much of an effect: generally, losing about 5% of your body weight within three months of taking orlistat is a sign that it’s working.
As part of our weight loss programme, you can get one-to-one support and advice from our team of expert coaches. They’ll be on hand to answer any of your questions.
Your weight management journey
A medicine’s side effects should definitely factor into your decision on whether to try it—but you should weigh these up against its potential benefits. And for orlistat, that could be losing at least 5% of your body weight.
Though orlistat isn’t the only weight loss medicine out there. Weight loss pens , including Mounjaro and Wegovy injections , can lead to weight loss of about 15 - 25% of your starting weight.
Each medicine has its own set of pros and cons: for instance, Wegovy side effects are common (though mild and usually clear on their own) too. And there are foods it’s probably best to avoid on Saxenda , another weight loss injector pen. Plus, weight loss pills and injections can differ in how effective they are.
Our clinicians will recommend which medicine they think is best for you when you sign up to our weight loss programme. As well as personalised guidance from our experts, your medication will be posted to you each month.
You can check if you’re eligible for the programme right now by filling out this short form.
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⁷“Gallstones: Overview.” NHS. www.nhs.uk/conditions/gallstones. Accessed 22 Jan. 2025.
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¹⁰Wilding, John P.H., et al. “Once-weekly Semaglutide in Adults with Overweight or Obesity.” The New England Journal of Medicine, vol. 384, no. 11, 2021:989-1002. www.nejm.org/doi/full/10.1056/NEJMoa2032183.
