Phentermine or Adipex is commonly used for weight loss in combination with diet and exercise. Although phentermine is one of the oldest FDA approved weight loss medications on the market there are still a lot of questions regarding its safety and efficacy. Does phentermine work long term?  Is phentermine necessary for weight loss? Is phentermine safe? All of these questions are valid. To answer these concerns along with how the most commonly prescribed weight loss medication in America aids in weight reduction we will explore a few key factors, starting with phentermine’s mechanism of action.

How Phentermine Works

Phentermine is a medication that is only available by prescription. It requires physician monitoring because it is categorized as a “controlled substance”. Phentermine is not a magic pill, it is simply a tool used to help with weight loss. The mechanism of action for phentermine causes a release of norepinephrine from the brain. Norepinephrine (also known as adrenaline) is a natural neurotransmitter produced and released in the body. This chemical is present when we are afraid, nervous, or involved in an action that requires energy, such as moderate to rigorous exercise or running from a bear (hopefully the latter never happens). As luck would have it, the release of norepinephrine from the brain also happens to promote several weight reducing activities such as appetite suppression, heightened attention and awareness, increased circulation of oxygen and nutrients throughout the body and increased metabolism. The increased oxygen circulation increases oxygen supply to the brain, which allows for rapid decision-making, and the activation of large skeletal muscles, which allows us to run from danger if necessary. This cascade of events is called the “fight or flight” response.

In addition to appetite suppression, norepinephrine reduces the thoughts, obsessions, and preoccupations with food. This is why some individuals taking phentermine may forget to eat because they don’t even think about food anymore. The metabolic increase from norepinephrine causes an increased use of stored body fat and an increased uptake of calories at rest in a 24-hour period.

If you are wondering why we don’t experience weight loss on a normal daily basis due to this presence of norepinephrine, it’s simply because norepinephrine only sticks around for a few seconds before it is broken down by the body.

Now that you understand how norepinephrine works, you also understand how phentermine works, it causes a release of norepinephrine and therefore turns on fat burning and counteracts the body’s natural resistance to weight loss.

How much weight will you lose on phentermine?

For all FDA approved weight loss medications, results will vary. On average individuals using phentermine can expect to lose twice the amount of weight that they would lose with diet and exercise alone. This means that if the average rate of weight loss is 8-10 pounds a month that a patient on phentermine may lose twice this rate, 16-20 pounds a month. This is of course considering that there are no counteractive medications, medical conditions, or other hindrances.  It is normal for the weight loss seen in the first few weeks to slow down to a more steady pace over time. This will occur even more for individuals closer to ideal weight. The most significant results, however, are those witnessed by the truly weight loss resistant. This is the individual who fails to lose weight despite multiple attempts over an extended period of several years. For this individual, phentermine can be the catalyst to finally turn on fat burning and when this occurs any weight reduction is extraordinarily significant!

What not to do on phentermine – the “Phentermine Commandments”

  • Thou shalt not skip meals
  • Thou shalt not severely restrict calories
  • Thou shalt not rush the process
  • Thou shalt not ignore nutrition and macronutrient balance
  • Thou shalt not ignore physical activity
  • Thou shalt not skip doctor’s appointments or routine monitoring


Despite the weight loss achieved while taking phentermine resist the urge to skip meals or severely restrict calories because this will only backfire long term. Also resist the urge to speed up weight loss results or create unrealistic demands. Keep in mind that phentermine is a weight loss tool, not a Genie in a Bottle, you cannot expect to lose all of your excess weight overnight.

What to expect when starting phentermine? Is phentermine safe?

Phentermine, like any medication, is safe when used for the right patient and when prescribed by a knowledgeable and experienced physician. Many medications can be refilled without routine check-ins with the doctor, however, phentermine is not one of those medications. Phentermine requires routine monitoring, physical examinations, vital sign checks, and regular office visits to discuss side effects.

What are the most common side effects for phentermine

The symptoms caused by phentermine will vary according to the patient but the most common side effects are appetite suppression and thirst. Thirst can occur as early as the first day and is easy to fix by simply drinking plenty of water. Other common symptoms are increased energy which can lead to insomnia or trouble sleeping. For this reason, it is recommended to take phentermine first thing in the morning. Thirst and insomnia typically subside after the first few weeks but for individuals with severe insomnia your doctor may decide to avoid phentermine altogether. Phentermine can also increase anxiety and other mental diseases such as bipolar disorder. Lastly, the most significant side effect of phentermine is an increase in heart rate and blood pressure along with heart palpitations. All of these symptoms along with other less common side effects not mentioned in this article must be monitored by a doctor.

Is phentermine addictive?

The short answer is not typically. Phentermine is in the class of medications known as amphetamines due to it’s chemical structure and action in the brain. However, it is not a chemical that patients typically get addicted to. Individuals who have successfully lost weight with phentermine may develop a psychological dependence, meaning they may emotionally feel fearful of regaining weight off the medication or feel like they are unable to make healthy decisions without taking phentermine. Some may even notice a physical increase in hunger and cravings once no longer taking the medication.

Will phentermine stop working over time?

It depends. For some people the answer is yes, after several months appetite suppression will subside.  For others, the effects of phentermine remain for a much longer period. Phentermine is not effective for everyone. If phentermine is ineffective for weight loss, meaning an extensive plateau, lack of appetite suppression, or minimal to no weight loss results over an adequate period, it should be discontinued. In this event, a weight loss doctor like Dr Covington can recommend an alternative treatment or FDA approved weight loss medication for you.

Phentermine requires a comprehensive weight management plan

This medication promotes weight reduction by providing energy, focus, suppressing food intake and reducing the desire to eat. Individuals undergoing a weight loss plan are able to make healthier food choices and pursue physical activity without hunger and fatigue. However, this medication is most effective when combined with a comprehensive weight management plan that includes medical oversight, nutrition, physical activity, and routine medical monitoring.

Want to learn more? Watch Dr Covington’s live video discussing phentermine and how it works.