Despite a myriad of options available on the internet and the high street, there is at present no miracle diet drug which is completely safe and clinically proven to get results without the aid of lifestyle changes, such as adopting a healthy diet and exercise programme.

The safety of diet pills is a subject that often makes the headlines. Two formerly available treatments, Rimonabant (Acomplia) and Sibutramine (Reductil), were withdrawn in the United Kingdom in 2008 and 2010 respectively, after they were both found to have dangerous side effects. Unlicensed diet pills have caused serious illness and even death in some cases.

Here, we’ll look at the potential side effects of diet pills which are available on prescription, at private slimming clinics and over the counter.

Not all of the side effects of the various treatments are listed here. Make sure you read the accompanying information with any product you buy or are prescribed before you use them. Before using any medication, it’s extremely important that you know what the potential side effects are before you start taking it, in case you need to seek medical assistance.

Orlistat (approved prescription weight loss medicine)

Only one prescription treatment is currently licensed for weight loss in the UK; Orlistat, also marketed as Xenical, works in the intestines by preventing enzymes in the digestive system from absorbing the fat in food, cutting fat intake by about a third. This fat is instead passed out of the body in fecal matter.

Orlistat or Xenical are only prescribed to patients who have a BMI (Body Mass Index) of at least 30 kg/m², or 28 kg/m² in those who are at increased risk of developing conditions such as hypertension or type 2 diabetes. It is only to be used in conjunction with a health and fitness programme and is normally issued when lifestyle changes alone have not proved effective.

A pill containing a smaller dose of orlistat, Alli, is available over the counter.

The common side effects of this treatment mostly affect the digestive system. They are generally mild, manifest themselves early on in the treatment and tend to subside after a short period, provided you stick to your prescribed diet and avoid meals high in fat. They can include:

  • Flatulence (sometimes with discharge)
  • Abdominal pain
  • More frequent and urgent visits to the toilet
  • Oily or soft stools
  • Diarrhoea
  • Pain in the rectum

Other common side effects are:

  • Headache
  • Disruption to the menstrual cycle
  • Low blood sugar (in patients with type 2 diabetes)
  • Fatigue
  • Disorders related to the teeth and gums

Patients with existing acute kidney conditions may develop renal stones, and women who use oral contraceptive pills are advised to use additional methods of contraception, should they be expedited from the body in cases of diarrhoea.

Some other side effects that have been reported while the drug has been available, though the frequency of these is not known, include: hepatitis; gallstones; pancreatitis; bleeding from the anus; diverticulitis; raised levels of liver enzymes and allergic reaction.

Mysimba (recently given approval)

Mysimba has recently received approval for use across the European Union and in the UK. The new weight loss treatment contains a combination of Bupropion and Naltrexone.

Both substances are also used to treat addictions. Bupropion increases the levels of dopamine in the brain, encouraging energy use and reducing appetite. Naltrexone helps to maintain weight loss and reduces cravings by affecting how food tastes.

Side effects of Bupropion/Naltrexone in studies conducted so far have been found to include:

  • Hypertension
  • Disturbed sleep patterns
  • Headaches
  • Difficulty concentrating
  • Heart palpitations
  • Flushing
  • Dizzy Spells
  • Muscle pain
  • Feeling sick

This treatment was granted a UK licence in 2017, therefore the product can be purchased with a prescription from GPhC-registered pharmacies.

Phentermine and Diethylpropion (not approved for weight loss in the UK)

While these medicines were technically legal at time of writing, they are not approved for weight loss in the UK and as such are not available on NHS prescription. Both phentermine and diethylpropion work as appetite suppressants, stimulating the nervous system and tricking the brain into not feeling sensations of hunger. They are part of the same family of drugs as amphetamines and were briefly made illegal in this country before the ban was overturned by legal action.

These drugs can be habit forming, and have been linked to high blood pressure and heart problems. Other side effects of these pills include:

  • Hyperactivity
  • Tremors
  • Difficulty sleeping
  • Vomiting
  • Diarrhoea
  • Mood swings
  • A dry or unpleasant tasting mouth

Because these drugs aren’t licensed in the UK for weight loss and may be potentially dangerous, we would strongly recommend against using them.

Herbal weight loss pills

There are an array of herbal treatments sold for weight loss. Some of them are accompanied by celebrity endorsements and expensive advertising campaigns, and are sold either as traditional herbal remedies or as containing extracts of natural ingredients such as seaweed, raspberry, cactus or dandelion root. However, clinical studies proving their efficacy are sparse.

Reported side effects of fat binding treatments in some cases are feelings of bloatedness, flatulence, constipation and diarrhoea.

Treatments with seaweed and dandelion root that claim to boost your metabolism make can your visits to the toilet more frequent, and other side effects may include diarrhoea, stomach ache and headaches.

Not much is yet known about the effects of raspberry ketones, but some users have reported feeling nervous and uneasy, an accelerated heartbeat and raised blood pressure.

Illegal diet pills

These must be avoided at all costs, as not only will the claims made regarding their benefits be spurious, but they may also contain banned substances and have very dangerous side effects. Because they have not been licensed by any medical body, it is impossible to determine their effectiveness or safety.

A number of deaths have been reported from the use of the pills known as DNP, which contain the industrial chemical dinitrophenol.

Just some of the side effects of these pills are:

  • Heart attack
  • Stroke
  • Kidney failure
  • Psychotic episodes
  • Vomiting
  • Fever
  • Accelerated heartbeat

Making the right choice

If you are looking to lose weight, your first port of call should always be your doctor. They will be able to advise you on the best course of action, taking into account your specific circumstances.

Prescription weight loss treatment can only be effective and therefore should only be used in conjunction with an exercise program and a controlled diet.

Take a look at our weight loss pages to find out more about the available treatments.

Page last reviewed:  20/04/2018