Phentermine vs. Sibutramine: Which Is Better?

Nowadays, excess weight is a widespread problem, which is growing because of quality of food and contemporary tendency in nutrition characterized by irregular and improper meals. A lot of medicines supporting weight loss achieved by physical exercises and diets have been created to solve the problem more efficiently.

Their main aim is to improve and keep the result, but still there are some differences. Here we are going to consider Phentermine and Sibutramine – two weight loss drugs, which are rather popular and often prescribed by dietitians.

Reasons for prescription

Phentermine and Sibutramine are prescribed when a patient has an abnormal body mass index (more than 27) in some cases accompanied with health problems that may affect the result of diet and physical activities.

Thus, a patient may get a prescription with one of these drugs in case of exogenous obesity. The difference is that:

  • Phentermine is strictly limited in the time of usage and shouldn’t be taken for more than three months;
  • Sibutramine can be used much longer – up to a year.

Normally, a patient prescribed with Sibutramine must be older than 16 and younger than 65. Phentermine, depending on the country, can be prescribed to:

  • Teens aged 16 and older, adults and geriatric patients in the USA;
  • Teens aged 12 and older, and adults younger than 65 in Australia and New Zealand.

If you do not belong to any of these age categories, these medicines are contraindicated to you. Other contraindications include the following:

  • pregnancy;
  • nursing;
  • cardiovascular diseases;
  • hypertension;
  • drug and alcohol abuse;
  • depressive disorders;
  • hypersensitivity to any ingredient.

Mechanism of action

Although Phentermine and Sibutramine are alike in their effect, the principles they work on are different.



  • Inhibits reuptake of Noradrenaline, Dopamine, and Serotonin through transporter reversal.
  • Inhibits reuptake of Noradrenaline, Serotonin and to a lesser degree Dopamine at the neuronal synapse.
  • Resulting in suppression of hunger signals and appetite decrease under the influence of a high level of these neurotransmitters.
  • A high level of Serotonin and Noradrenaline promotes a sense of satiety and decrease in appetite, thereby reducing food intake.
  • It is theorized that Phentermine may indirectly increase leptin levels in the brain.
  • It is suggested that Sibutramine can stimulate thermogenesis.
  • “Satiety hormone” signals the brain of saturation and the need to stop eating food.
  • Increased thermogenesis leads to enhancement of resting energy expenditure and weight reduction.

Rules of usage

Both Phentermine and Sibutramine are addictive substances, so it’s necessary to follow the prescription given by your doctor. It’s strictly forbidden to increase the dose, to take it more often than your doctor tells you or continue taking the medicine after the end of your course.

Phentermine is produced as immediate release tablets and capsules, as well as extended-release capsules. Sibutramine is produced as capsules exclusively.

Phentermine is tightly bound with meals and can be taken:

  • once a day in the morning;
  • two times a day in the morning and early afternoon;
  • three times a day, half an hour before your meals.

Sibutramine is not so sensitive in relation to food and must be taken once a day, in the first half of the day either during your meals or without it.

Side effects

Like most of drugs, Phentermine and Sibutramine have some side effects, and it’s necessary to be aware of what the medications’ action may be accompanied with.

There are some possible common side effects for Phentermine and Sibutramine:

  • dry mouth;
  • constipation;
  • insomnia;
  • vomiting;
  • dizziness;
  • restlessness;
  • heart palpitation;
  • headache.

Moreover, there are some individual side effects for each drug, caused by difference in the nature of their action. For example, Sibutramine may cause nightmares, peripheral edema, while Phentermine – euphoria followed by fatigue and depression.

In general, the number of side effects of Sibutramine is much bigger than that of Phentermine. Moreover, cardiovascular diseases initiated by Sibutramine are highly frequent and in some cases have already led to fatal outcomes. That’s why being less effective for weight loss, Phentermine is more preferable in obesity treatment.