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Phentermine For Weight Loss: Who Can It Be Prescribed to and Why? 

8 min read
white and blue phentermine-like capsules_phentermine for weight loss
Melissa Mitri post Reviewer Melissa Mitri post Reviewer
Verified by Melissa Mitri
MS, Registered Dietitian, Former President of CT Academy of Nutrition & Dietetics

Phentermine weight-loss pills can help you lose weight by reducing appetite. Learn about phentermine effects, who and when can be prescribed.

Table of Contents

Obesity has become a global crisis, with about two billion people estimated to be overweight or obese by 2030. [1] The dream of attaining a healthy weight is one that many people share. 

If you are looking to lose weight, it’s important to be well-equipped with the right tools. An appetite suppressant may help make your weight-loss journey easier, and one such medication is phentermine weight-loss pills.

Phentermine 101

In this section, we discuss what phentermine is, its therapeutic effects and associated risks, and to whom it can be prescribed and for what.

phentermine for weight loss facts described - fda approval, prescription, effects, how long to take_

What is Phentermine?

For people with severe obesity or obesity complicated by comorbid diseases, medications such as phentermine may be prescribed as an additional treatment to lifestyle modification therapy.

Phentermine is a diet aid approved for use by the US Food and Drug Administration in 1959. [2]

Brand names of phentermine include Ionamin, Fastin, Zantryl, Adipex-P, and Obe-Nix-30. 

Phentermine’s chemical structure is similar to the stimulant amphetamine, and it’s available by prescription only.

​Phentermine weight-loss pills are one of the most prescribed weight-loss medications because they are considered safe, effective, and widely available. [3] Phentermine is also a common ingredient of lipotropic B12 injections for weight loss.

Does Phentermine Burn Fat?

How does phentermine work, and how does it achieve the weight loss it promises? 

You may be surprised to know that phentermine does not directly burn fat. Instead, it targets the body’s basic weight-regulating functions.

Phentermine belongs to a class of drugs called anorexiants, which act on the parts of the brain that control appetite. It works to produce certain neurotransmitters like dopamine and norepinephrine, which heighten feelings of satiety. [4] 

When this happens, the body receives a signal from the brain indicating that it is already full, making you less likely to eat.

The release of dopamine, dubbed the “happy hormone,” is especially beneficial for those who have a tendency for emotional eating or stress eating. This neurotransmitter produces the feelings of reward and gratification usually experienced from food consumption.

Where Can I Get Phentermine to Lose Weight?

Phentermine is a prescription drug available in most DEA-registered pharmacies nationwide.

However, because of its stimulating effects on the central nervous system, phentermine weight-loss pills can be addictive, which is why it is not available over the counter.

How Long Can You Take Phentermine?

Phentermine is usually prescribed by doctors for weight loss as a short-term adjunct to lifestyle modification. That means it is generally taken for around 8 to 12 weeks.

Through appropriate use of this drug combined with a healthy exercise and diet regimen, weight loss can occur.

How Fast Does Phentermine Work?

Phentermine reaches peak concentrations in the bloodstream 3 to 4.4 hours (180 to 264 minutes) after administration. At that point in time, you will feel the appetite-suppressing effects of the drug the most.

For maximum absorption and efficacy, phentermine is taken on an empty stomach, usually 30 minutes prior to meals.

How Much Weight Can You Lose In A Month With Phentermine?

On average, people lose 3 to 5 pounds in a month on phentermine. However, this number is inconsistent throughout the medication period, as phentermine shows greater weight loss effects when taken for a prolonged time of up to a year. [5] [6] [9]

The typical amount of weight loss with phentermine has not been conclusively established. This is because clinical studies have shown other variables can influence the amount of weight loss that occurs while taking phentermine.

Some of those other influences included the population studied, the type of diet they were following, or the researcher investigators involved in the study.  

Varying amounts of weight loss may depend on the dosage or whether phentermine is prescribed along with other therapies or medications. If a certain percentage of weight is not lost at a specific point in time, your physician may either discontinue the medication or increase the dose.

How Much Weight Can You Lose In A Week With Phentermine?

You may expect to lose 0,75 to 1,25 pounds per week on phentermine. However, be prepared not to see a result during the first couple of weeks of treatment, as phentermine needs time to build up in the system and achieve the necessary concentration to kick in.

When Is Phentermine Prescribed?

You need a full doctor’s consultation and evaluation of the risks versus benefits before you can get prescribed phentermine.

Phentermine for Weight Loss

Doctors typically prescribe phentermine for patients with 

  • a body mass index (BMI) equal to or greater than 30 kg/m2,
  • a BMI equal to or greater than 27 kg/m2 with obesity-related complications such as diabetes, hypertension, or high cholesterol levels. [7]

The dosage is adjusted based on your response to the drug, with the goal being to provide the desired benefits with the lowest effective dose possible. 

Phentermine is usually prescribed for people with simple or exogenous obesity. Exogenous obesity is a form of obesity brought about by food consumption that is disproportionate to the person’s activity level, resulting in increased fat storage.

Because this drug’s clinical effect is appetite reduction, it has been observed to work better in people with less cognitive control over their eating habits.

Special Considerations

Phentermine is not approved for children age 16 or younger. Pediatric obesity is a complex condition with multiple subtypes, often requiring long-term medication. 

It is also contraindicated in pregnant and nursing mothers because of its similarity to amphetamine in how it works, which may result in adverse fetal outcomes. 

Older patients and patients with kidney impairment should use phentermine with caution.

Phentermine Side Effects and Precautions

During your initial consultation with your physician, make sure to tell them about your medical history and any other medications you are currently taking.

If you have a history of cardiac disease, you may be prescribed another anti-obesity drug rather than phentermine. 

The same applies if you are currently taking an antidepressant like an MAOI, SSRI, SNRI, or TCA, as concurrent administration of these drugs with phentermine may be fatal.

Common side effects of taking phentermine include headaches, restlessness, irritability, nausea, and vomiting.

Phentermine side effects in females may include menstrual upset and an increased risk of depressive symptoms. 

While rare, severe side effects are possible, such as increased blood pressure, suicidal thoughts, metabolic acidosis, vision problems, cognitive impairment, and hypokalemia (low potassium levels). [8] 

If you experience any of these side effects, inform your doctor or pharmacist as soon as possible.

A tolerance to the drug may also manifest a few weeks into treatment. When this occurs, it means phentermine lost some of its effectiveness over time as the body adjusts to its effects. If you notice tolerance, notify your doctor, but continue to take the medication as prescribed until further guidance is issued.

Avoid taking this medication in the late evening because it may cause insomnia. Make sure to monitor your weight at least three times per week while wearing the same amount of clothing and at the same time of day. 

A weight-loss app will be incredibly beneficial for tracking your weight and your progress toward your goals.  

Can You Lose Weight on Phentermine Only?

Phentermine could be a potential tool in your weight-loss toolbox, but it alone won’t bring about weight loss. Remember, phentermine weight-loss pills do not directly burn fat, they only curb appetite. 

It may be tempting to skip meals to speed up the weight-loss process. It may also be tempting to try out topical products such as ginger oil, which similarly promise weight reduction with little effort on your part, or dietary supplements such as snap packets and ballerina teas

However, doing so might actually result in more harm than good. When you do not get enough nutrients in a day, your body shifts into “survival mode” and deposits fat rather than burning it for energy in order to ensure your continued survival. 

As such, proper diet and exercise remain the cornerstones of obesity prevention and treatment.

Final Words

The use of phentermine for weight loss is considered generally safe and effective. It can provide bigger results when used in combination with lifestyle modification practices, such as proper diet and exercise. It’s important to always evaluate the pros and cons of any medication to determine what’s best for your needs.

As such, we’ve listed some key points to remember when taking this drug.

  • Phentermine does not directly burn fat but instead targets the body’s basic weight-regulating functions.
  • During your initial consultation with a physician, tell them about your history of diseases and any medications you are taking. 
  • Take this drug exactly as prescribed. If you experience any signs of an allergic reaction, immediately seek medical attention.
  • Remember to weigh yourself regularly and update your physician on your progress. If a certain percentage of weight is not lost at a specific time, your physician can discontinue the medication or increase the dose. 

If you experience difficulty breathing, dizziness or lightheadedness, palpitations, or loss of consciousness, seek medical attention right away as this may indicate an allergic reaction to phentermine. 

If you are curious about phentermine for weight loss, speak to your doctor about the pros and cons for your personal situation.

Disclaimer This article is intended for general informational purposes only and does not address individual circumstances. It is not a substitute for professional advice or help and should not be relied on to make decisions of any kind. Any action you take upon the information presented in this article is strictly at your own risk and responsibility!

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