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Reviewed on Jun 10, 2024. by Dr Daniel Atkinson GP Clinical Lead Registered with GMC (No. 4624794) Next review due on Jun 10, 2026.
Daniel

Last updated on Jun 10, 2024.

Side effects of weight loss treatments

First of all, it’s important to know that using a treatment isn’t a shortcut to weight loss. Those who choose to take them will still need to commit to making healthy food choices and taking part in regular physical activity. It’s also important to be aware that all medications, no matter what they’re treating, have the potential to cause side effects. Since everybody is different, which side effects (if any) you may experience and how severe they can be will vary.

Some of the most common side effects associated with taking weight loss treatments include:

  • Nausea
  • Constipation or diarrhoea
  • Vomiting
  • Headaches
  • Insomnia
  • Dry mouth

Before commencing a course of any medication it’s a good idea to familiarise yourself with the potential side effects by reading the patient information leaflet.

Orlistat side effects

Orlistat is one of the most well-known weight loss drugs and is the generic name for this medication. Orlistat is also the name of the active ingredient in several weight loss medications.

Orlistat works by altering the way that fat is absorbed by the body, preventing around a third of the fat from the food you eat being absorbed. Instead, it is passed out through your digestive system when you empty your bowels. This means that while it is effective in preventing you from gaining weight, you won’t necessarily lose weight unless you also follow a calorie-deficit diet and take regular exercise. A clinical study showed that after a year, patients who took orlistat lost significantly more weight than those who took a placebo.

But is Orlistat safe? Orlistat is considered to be extremely safe and can be a suitable weight loss aid for those within the outlined BMI perimeters. A doctor should assess whether it’s suitable for you when it’s prescribed at a 120mg dosage.

  • Very common Orlistat side effects

  • Common Orlistat side effects

  • Unknown frequency Orlistat side effects

(affects more than 1 in 10 users)

  • Headache
  • Abdominal pain/discomfort
  • Oily discharge
  • Oily or fatty stools
  • Liquid stools
  • Urgent or increased need to open the bowels
  • Flatulence with discharge
  • Low blood sugar levels (experienced by some people with type 2 diabetes).

(affects 1 to 10 users out of 100)

  • Rectal pain or discomfort
  • Tooth or gum disorder
  • Irregular menstrual cycle
  • Tiredness
  • Soft stools
  • Incontinence (stools)
  • Bloating (experienced by some people with type 2 diabetes).
  • Allergic reactions - with symptoms such as itching, rash, wheals, severe difficulty in breathing, nausea, vomiting and feeling unwell
  • Skin blistering
  • Diverticulitis
  • Bleeding from the rectum
  • Increases in the levels of some liver enzymes may be found in blood tests
  • Hepatitis (inflammation of the liver) - with symptoms such as yellowing skin and eyes, itching, dark coloured urine, stomach pain and liver tenderness (sometimes with loss of appetite). Stop taking Orlistat and inform your doctor
  • Gallstones
  • Pancreatitis (inflammation of the pancreas)
  • Oxalate nephropathy (build up of calcium oxalate which may lead to kidney stones)
  • Vitamin deficiency - some fatty foods can be rich in certain vitamins therefore reducing fat in the diet or not absorbing fats can lead to deficiencies. This can be checked on by having a blood test to look at vitamin levels a couple of times a year.

Alli side effects

Alli is an over-the-counter version of Orlistat and is an effective drug for weight loss. Alli can be purchased without a prescription as it contains a much smaller dose of the active ingredient, at just 60mg per dose rather than the 120mg prescription for Orlistat.

Alli also works in the same way, restricting the amount of fat that is absorbed by the body from the foods that you eat. When combined with a calorie-controlled diet and exercise, Alli can help patients to lose more weight than through dieting alone.

Despite being a lower dose than Orlistat, patients who take Alli should be aware that they are still at risk of experiencing side effects.

  • Very common Alli side effects

  • Common Alli side effects

  • Serious Alli side effects

(may affect more than 1 in 10 users)

  • Fatty or oily stools
  • Flatulence, with or without oily spotting
  • Sudden bowel motions
  • Soft stools.

(may affect up to 1 in 10 users)

  • Stomach pain
  • Incontinence (stools)
  • More frequent bowel motions
  • Liquid stools
  • Anxiety.

(it is unknown how likely these are to occur)

  • Allergic reaction symptoms can include severe difficulty breathing, swollen face, rapid heart beat, sweating, rash, itching and collapse
  • Bleeding from the rectum
  • Pancreatitis (inflammation of the pancreas). Symptoms may include severe abdominal pain sometimes radiating towards the back, possibly with fever, nausea and vomiting
  • Diverticulitis (inflammation of the large intestine). Symptoms may include lower stomach (abdominal) pain, particularly on the left side, possibly with fever and constipation
  • Skin blistering (and/or blisters that burst)
  • Gallstone causing severe stomach pain
  • Oxalate nephropathy (build up of calcium oxalate which may lead to kidney stones)
  • Hepatitis (inflammation of the liver). With symptoms such as yellowing skin and eyes, itching, dark coloured urine, stomach pain and liver tenderness, sometimes with loss of appetite
  • Vitamin deficiency.

Xenical side effects

Xenical is a prescription brand weight loss drug; the active ingredient is Orlistat. This means that this medication works in exactly the same way and the side effects that you could experience remain the same.

  • Very common Xenical side effects

  • Common Xenical side effects

  • Unknown frequency Xenical side effects

(affects more than 1 in 10 users)

  • Headache
  • Abdominal pain/discomfort
  • Oily discharge
  • Oily or fatty stools
  • Liquid stools
  • Urgent or increased need to open the bowels
  • Flatulence with discharge
  • Low blood sugar levels (experienced by some people with type 2 diabetes).

(affects 1 to 10 users out of 100)

  • Rectal pain or discomfort
  • Tooth or gum disorder
  • Irregular menstrual cycle
  • Tiredness
  • Soft stools
  • Incontinence (stools)
  • Bloating (experienced by some people with type 2 diabetes).
  • Allergic reactions - with symptoms such as itching, rash, wheals, severe difficulty in breathing, nausea, vomiting and feeling unwell
  • Skin blistering
  • Diverticulitis
  • Bleeding from the rectum
  • Increases in the levels of some liver enzymes may be found in blood tests
  • Hepatitis (inflammation of the liver) - with symptoms such as yellowing skin and eyes, itching, dark coloured urine, stomach pain and liver tenderness (sometimes with loss of appetite)
  • Stop taking Orlistat and inform your doctor
  • Gallstones.
    Pancreatitis (inflammation of the pancreas)
  • Oxalate nephropathy (build up of calcium oxalate which may lead to kidney stones)
  • Vitamin deficiency.

Mysimba side effects

Mysimba is a fairly new treatment for weight loss that works a little differently to Orlistat. Mysimba targets the areas of the brain that regulate appetite, reducing food cravings and suppressing feelings of hunger. This makes it easier for patients to stick to a healthy diet and remain on a successful weight loss program. It is only available by prescription and contains active ingredients naltrexone and bupropion.

As with any other weight loss medication, there are a range of side effects associated with taking Mysimba.

  • Very common Mysimba side effects

  • Common Mysimba side effects

  • Uncommon Mysimba side effects

  • Rare Mysimba side effects

(may affect more than 1 in 10 users)

  • Constipation
  • Headache
  • Nausea
  • Vomiting.

(may affect up to 1 in 10 users)

  • Hot flush
  • Increased blood pressure (sometimes severe)
  • Pain in the upper part of the abdomen
  • Pain in the abdomen
  • Excessive sweating (hyperhidrosis)
  • Rash, itching (pruritus)
  • Hair loss (alopecia)
  • Irritability
  • Feeling jittery
  • Anxiety
  • Dizziness (vertigo)
  • Feeling shaky (tremor)
  • Difficulty in sleeping (avoid taking Mysimba near to bedtime)
  • Changes in the taste of food (dysgeusia) or dry mouth
  • Difficulty concentrating
  • Feeling tired and sleepy, drowsy or a lethargic
  • Ringing in the ears (tinnitus)
  • Fast or irregular heartbeat.

(may affect up to 1 in 100 users)

  • Hives (urticaria)
  • Hypersensitivity
  • Difficulty in getting or keeping an erection
  • Feeling abnormal, weakness (asthenia)
  • Thirst, feeling hot
  • Chest pain
  • Increased appetite and/or weight gain
  • Abnormal dreams
  • Feeling nervous, feeling spacey, tension, agitation, mood swings,
  • Tremor of the head or a limb which increases when trying to perform a particular function (intention tremor)
  • Balance disorder
  • Loss of memory (amnesia)
  • Tingling or numbness of the hands or feet
  • Motion sickness
  • Burping
  • Abdominal discomfort
  • Indigestion
  • Inflammation of the gallbladder (cholecystitis)
  • Increased creatinine levels in the blood (indicating loss of kidney function)
  • Increased liver enzymes and bilirubin levels, liver disorders.

(may affect up to 1 in 1000 users)

  • Low amount of certain white blood cells (Lymphocyte count decreased)
  • Decreased haematocrit (indicating loss of red blood cell volume)
  • Swelling of eyelids, face, lips, tongue or throat, which can cause great difficulty in breathing (angioedema)
  • Hernia
  • Toothache
  • Dental cavities
  • Pain in the lower part of the abdomen
  • Injury to the liver due to drug toxicity
  • Jaw discomfort
  • A disorder characterised by a sudden need to urinate (micturition urgency)
  • Irregular menstrual cycle, vaginal bleeding, dryness of the female vulva and vagina
  • Excessive loss of body water (dehydration)
  • Hallucinations
  • Fainting, feeling faint or loss of consciousness
  • Fits
  • Passage of fresh blood through the anus usually in or with stool (haematochezia)
  • Coldness of extremities (hands, feet).

Side effects of weight loss injections

Weight loss injections are an alternative to diet pills. There are several options to choose from, including Saxenda and Wegovy. These work using appetite suppression. Weight loss injections can help users keep a feeling of a satisfied appetite for longer by altering their feelings of hunger. 

Liraglutide side effects

Liraglutide was the first weight loss injection to hit the market, under the brand name Saxenda. Now there are a few more versions of it available (Nevolat, Biolide, Plaobes), though they all work in the same way. Like all GLP-1 weight loss treatments, it works by regulating your appetite, which can lead to eating fewer calories and losing weight. However unlike Wegovy and Mounjaro, which are used once a week, liraglutide injections are used daily. It should be used in conjunction with a low calorie diet and regular exercise.

But is Liraglutide safe? It has received approval to be used as a weight loss aid but like all other weight loss drugs, it can cause side effects.

  • Very common Liraglutide side effects

  • Common Liraglutide side effects

  • Uncommon Liraglutide side effects

  • Rare Liraglutide side effects

  • What are the side effects of stopping Liraglutide?

(may affect more than 1 in 10 users)

  • Diarrhoea
  • Constipation
  • Feeling sick
  • Vomiting.

(may affect up to 1 in 10 users)

  • Bruising, pain or irritation and the injection site
  • Stomach problems such as indigestion, pain, heartburn, feeling bloated, wind, burping and a dry mouth
  • Feeling lethargic
  • Change in the sense of taste
  • Feeling dizzy
  • Insomnia
  • Gallstones
  • Low blood sugar (hypoglycaemia)
  • Increase of pancreatic enzymes, such as lipase and amylase.

(may affect up to 1 in 100 users)

  • Feeling generally unwell
  • Dehydration
  • Delay in the emptying of the stomach
  • Inflamed gallbladder
  • Allergic skin rash
  • Faster pulse.

(may affect up to 1 in 1000 users)

  • Reduced kidney function
  • Acute kidney failure.

Once you have achieved your weight loss goals, you’ll probably want to stop taking Liraglutide and try to maintain your weight yourself. Alternatively, you may want to stop taking Liraglutide if it isn’t proving as effective for you as you'd like, and you want to try another weight loss drug.

There are no specific side effects associated with stopping Liraglutide, but unless you follow a strict diet and exercise plan, you may find that you regain the weight that you have lost.

Current NICE guidance recommends limiting treatment to two years, however, there is an acknowledgement that continuing treatment beyond this can be useful for many patients who find that doing so helps maintain weight loss. Once weight loss goals are achieved, the dosage can be reduced to maintain weight.

Wegovy side effects

Wegovy is an injection that’s used to treat weight loss. It works using appetite suppression. Currently, it’s only available as a brand-name medication and not in a generic form. Wegovy should be used alongside a healthy diet and exercise program to be as effective as possible at helping you to lose weight.

  • Very common Wegovy side effects

  • Common Wegovy side effects

  • Uncommon Wegovy side effects

  • Rare Wegovy side effects

(may affect more than 1 in 10 users)

  • Feeling sick
  • Headache
  • Constipation
  • Diarrhoea.

(may affect up to 1 in 10 people)

  • Vomiting
  • Dizziness
  • Low blood sugar
  • Indigestion
  • Inflamed stomach (‘gastritis’)
  • Reflux or heartburn
  • Stomach pain
  • Bloating
  • Constipation
  • Burping
  • Gallstones
  • Less appetite
  • Flatulence.

(may affect up to 1 in 10 users)

  • fast heartbeat
  • increase of pancreatic enzymes (such as lipase and amylase)
  • delayed emptying of the stomach.
  • Severe allergic reactions (anaphylactic reactions, angioedema) which require immediate medical attention.

Mounjaro side effects

Mounjaro (tirzepatide) is the newest type of weight loss injection. Unlike liraglutide and semaglutide, which target the GLP-1 hormone to promote weight loss, Mounjaro targets GIP and GLP receptors. In clinical trials, this has shown it to produce improved weight loss results. It’s used weekly, alongside a diet and exercise plan.

  • Very common Mounjaro side effects

  • Common Mounjaro side effects

  • Uncommon Mounjaro side effects

(may affect more than 1 in 10 users)

  • Low blood sugar (when used for type 2 diabetes in conjunction with other blood sugar lowering medicines)
  • Nausea
  • Diarrhoea
  • Vomiting
  • Stomach pain
  • Constipation

(may affect up to 1 in 10 people)

  • Mild allergic reactions
  • Dizziness
  • Low blood pressure
  • Indigestion
  • Bloating
  • Burping
  • Gas
  • Heartburn/acid reflux
  • Hair loss
  • Fatigue
  • Injection site reactions
  • Fast heart rate
  • Gallstones

(may affect up to 1 in 10 users)

  • Injection site pain
  • Gallbladder infection
  • Altered sense of taste
  • Change in skin sensation

Which weight loss drugs aren't safe?

The weight loss drugs listed above have all undergone extensive clinical trials to prove their safety and efficacy.

Taking unlicensed weight loss medications can have serious consequences for your health, as they may contain inactive or even dangerous ingredients.

To guarantee taking safe weight loss pills, consult with your doctor first. They will assess your suitability for weight loss drugs and recommend safe weight loss tablets or injections based on your individual needs.

When not to take weight loss drugs

Not everyone who is overweight or obese is automatically a suitable candidate to take weight loss drugs. For this reason, patients should consult with their doctor before starting any new medication, including those used for weight loss. They will ensure that the benefits of taking weight loss medication will outweigh the risks associated with the treatment.

Typically, patients who are not good candidates for weight loss drugs are those who are pregnant or nursing, under the age of 18 and have a BMI of less than 27.

Depending on the exact weight loss drug you choose, there may also be further factors to consider. For example, Orlistat is not recommended for patients who have conditions where food isn’t absorbed properly, or those taking certain medications like ciclosporin or warfarin. A doctor will be able to discuss medication interactions with you at your consultation.

Should I use weight loss drugs?

Choosing to use weight loss drugs to help you achieve your weight loss goals can feel like a big decision. It’s important to take the potential side effects into account.

If you want to explore the different ways you can lose weight, then it’s a good idea to explore all your options.

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This page was medically reviewed by Dr Daniel Atkinson, GP Clinical Lead on Jun 10, 2024. Next review due on Jun 10, 2026.

Last updated on Jun 10, 2024.

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