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Phentermine: How It Affects Females + Side Effects To Watch Out For

a woman with light dumbbells
a woman with light dumbbells

While newer medications such as GLP-1 receptor agonists have recently become popular for their rapid weight loss effects, another prescription weight loss medication known as phentermine has actually been around for more than 60 years.

Phentermine is one of the oldest weight loss drugs on the market, and while it has proven effective in promoting weight loss, it is crucial to acknowledge potential side effects associated with its use, especially in females.

Read on to learn more about phentermine, how it can support weight management and the important considerations to keep in mind when it comes to side effects. 

What is Phentermine?

phentermine pills
Source: Pexels

Phentermine is a prescription medication that is used to promote weight loss. It belongs to the drug class called sympathomimetic amines, which are more commonly known as stimulant drugs. 

Phentermine is authorized for short-term use for up to 12 weeks due to the potential side effects that have been associated with long-term use. In 2012, the Food and Drug Administration (FDA) approved a combination of phentermine-topiramate (brand name Qsymia) for the treatment of long-term obesity with at least one weight-related condition.

Phentermine is considered a “Schedule IV” controlled substance in the United States, meaning that it is subject to prescribing restrictions and is not available over the counter.

Phentermine Dosage

Phentermine comes in different strengths and different forms: capsules, extended-release tablets, oral dissolving tablets, and tablets. It’s sold under brand names Lomaira and Adipex-P.

When your doctor prescribes you with phentermine, the typical dosages usually range from 15 to 37.5 milligrams per day depending on the form you take. 

For adults and children who are 17 years old and older, the recommended dosage of Adipex-P capsules is 37.5 milligrams once a day, approximately two hours after breakfast. The typical dose of Lomaira tablets is eight milligrams, taken three times a day before each meal.

What Does Phentermine Do To The Body?

how phentermine works

Phentermine is a stimulant, and is structurally similar to amphetamines. As a result, it can be addictive and may carry a risk of misuse. Because phentermine is a stimulant, it may impact mood, metabolism, and energy throughout the day, which can result in weight loss. 

Phentermine may also work as an appetite suppressant by raising the levels of certain chemicals in your brain that regulate your appetite. However, there are still many unknowns at this time, and more research is needed to understand the exact mechanisms that link phentermine and weight loss.

While phentermine can have desired effects such as decreased hunger and weight loss, there are potential side effects that must be considered before taking a stimulant medication. This class of medications can cause serious side effects for people who have:

  • Heart or lung disease
  • Extremely high blood pressure
  • Low blood sugar (while taking diabetes medications)

Does Phentermine Work for Weight Loss?

Yes, phentermine is a prescription drug that can be effectively used as part of a complete weight management program. However, phentermine’s effectiveness may decrease over time. It’s important to create healthy lifestyle habits as part of a holistic weight loss program. A successful weight management program commonly includes:

  • Learning how to set realistic goals
  • Eating a healthy diet and monitoring calories 
  • Getting appropriate exercise
  • Reducing stress and prioritizing sleep

Phentermine vs. Semaglutide: What’s the Difference?

phentermine vs semaglutide
Source: Pexels

Semaglutide is the active ingredient in the GLP-1 drugs Ozempic, Wegovy, and Rybelsus. Ozempic and Wegovy are injections that are given weekly, while Rybelsus is a pill that is taken daily. 

Phentermine is also an oral medication, but can only safely be used for 12 weeks according to the FDA. GLP-1 medications such as Ozempic and Wegovy were originally created for patients with type 2 diabetes, though they have both become popular as weight loss medications and can be used short-term or long-term. 

Wegovy is the only GLP-1 medication specifically approved for weight loss. However, this medication can be very expensive if your insurance company does not cover the cost of the medication. 

Some natural supplements may help you lose weight such as berberine and resveratrol. Berberine can be found in Goldenseal and Oregon Grape, and has been used in Chinese medicine for weight loss for years. While some studies have indicated that berberine may help with weight loss, more research is still needed. 

Common Side Effects of Phentermine

common side effects of phentermine

There are several common side effects of phentermine to be aware of, and people who have heart disease, kidney disease, or uncontrolled high blood pressure should check with their healthcare provider before taking phentermine. 

Serious side effects can include decreased blood flow to your heart, high blood pressure, and psychosis. Other adverse effects of phentermine can include any of the following:

  • Chest pain
  • Decreased ability to exercise
  • Dizziness or drowsiness
  • Fainting
  • Dry mouth
  • Tachycardia, irregular heartbeat, or palpitations
  • Headache
  • Constipation
  • Numbness or tingling in the arms or legs
  • Swelling of the feet or lower legs
  • Trembling or shaking of the legs, arms, hands, or feet
  • Trouble breathing
  • Trouble sleeping
  • Trouble with thinking, speaking, or walking
  • Weakness

It’s also important to be mindful of potential drug interactions, as certain medications that belong to the drug class MAOIs, such as isocarboxazid (Marplan), phenelzine (Nardil), selegiline (Emsam), or tranylcypromine (Parnate) may cause critically high blood pressure when taken with phentermine.

Other medications for depression, citalopram (Celexa), escitalopram (Lexapro), fluoxetine (Prozac), and sertraline (Zoloft) can cause fast heart rate, high blood pressure, dysphoria, overstimulation, and insomnia when paired with phentermine.

According to medical professionals, phentermine should not be taken by people with:

  • A history of drug abuse or drug addiction
  • Uncontrolled high blood pressure
  • Overactive thyroid, glaucoma
  • A history of heart attack or stroke
  • Heart disease 

If you are 65 or older, talk to your healthcare provider about other weight loss medications. Finally, if you are pregnant or breastfeeding, have any of the above medical conditions, or have a history of an eating disorder such as anorexia or bulimia, you should discuss other treatment options with a healthcare professional.

How Does Phentermine Affect Females? 

phentermine in women
Source: Pexels

For women, phentermine can have some specific side effects to keep in mind. This weight loss drug is known to reduce caloric intake and lead to weight loss, which may have an impact on a woman's menstrual cycle

Losing 10 percent or more of initial body weight and reducing caloric intake has been shown to cause menstrual irregularities in women. Varying levels of calorie restriction have been shown to impact menstruation, and if phentermine results in significant calorie deprivation, that can lead to menstrual imbalances. 

However it's important to emphasize that these studies haven't shown a direct link between phentermine and menstrual cycle imbalances. That's likely because  phentermine is only prescribed for short-term use, and a woman's menstrual cycle isn't acutely impacted by energy deficits. 

In other words, chronic deprivation and body weight changes will impact menstruation. Acute deprivation may not have a significant impact.

Other studies suggest that phentermine can lead to changes in libido in women. It’s crucial for women considering phentermine to consult with their healthcare providers and seek medical advice, as individual responses may vary, and the medication should be used cautiously, especially in those with pre-existing health conditions.

Healthy Weight Loss Strategies to Consider

Weight loss medications are only one part of a comprehensive weight loss program. Achieving sustainable and healthy weight loss involves adopting well-rounded strategies built around:

Additionally, monitoring dietary choices and their impact on your glucose levels can help support your weight loss goals. Continuous glucose monitoring can be a valuable tool for people seeking effective weight loss strategies. The real-time glucose insights provided by a CGM can contribute to a more personalized approach to building healthy lifestyle habits, empowering you to make optimal choices for both your diet and glucose management.

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Victoria Eaton, RDN, LDN, IFNCP

Reviewed by: Victoria Eaton, RDN, LDN, IFNCP

Victoria began her career in the functional medicine space in 2015. She has extensive experience interpreting labs and supporting weight loss, gut imbalances, and chronic migraines. She received her Bachelor of Science degree in nutrition and dietetics from Missouri State University.