It seems like news about medical weight loss is everywhere these days. From celebrities to your next-door neighbor, taking a prescription to lose weight seems less of a hushed topic than in the past.
Weight loss medications aren't new, but they all work differently in the body. One that's been around for a while is Phentermine. Phentermine is an FDA-approved medication that suppresses appetite, so calorie intake drops. A pill that makes you less likely to overeat may sound enticing, especially for people who've tried to lose weight without success.
Still, it's essential to understand the potential side effects of Phentermine before taking it. This is especially true for women, as women's bodies are especially sensitive to rapid weight loss and restriction.
Here's what you need to know about Phentermine's side effects and why taking it for weight loss is more complex than it may seem.
What is Phentermine?
Phentermine is part of a class of drugs known as anorectics or anorexigenics prescribed to facilitate weight loss by acting as an appetite suppressant. It's available in several forms, including tablets and capsules. Phentermines' chemical structure is similar to amphetamines (and also carries similar risks of addiction), so it's considered a controlled substance by the FDA. It has brand names like Adipex-P and Lomaira and lower-cost generic options.
In the 90s, "fen-phen," a combination of Phentermine and fenfluramine, was sold as a quick fix for weight loss. However, it was quickly taken off the market after being linked to heart valve damage and fatal pulmonary hypertension.
Phentermine alone does not appear to have the same severe side effects, so it's still used and prescribed by physicians for suppressing the appetite (although there is still a potential for serious side effects, as you'll learn below).1
What is Phentermine Used For?
Phentermine is approved by the FDA for people with a BMI over 30 (obesity) or those with a BMI above 27 (overweight) with additional health problems like high cholesterol, hypertension, or diabetes. This means it's not a drug that should be given to people who want to lose a few pounds. It's also not meant to be used long-term (more than 8 to 12 weeks) and is usually prescribed with dietary changes and exercise recommendations.
Phentermine is not recommended for people with certain medical conditions, including:3
- Heart disease
- Uncontrolled high blood pressure
It's also unsafe for people with a history of drug abuse or anxiety because of its similarity to amphetamines.
Phentermine for Weight Loss: How it Works
The way Phentermine works for weight loss isn't completely clear, but experts believe there are two primary mechanisms. First, Phentermine may increase the production of neurotransmitters (chemical messengers in the brain) like dopamine and norepinephrine that keep food cravings and appetite to a minimum.
Second, it lowers appetite by activating the nervous system to release chemicals called catecholamines that influence appetite and metabolism. Metabolic activity, including breathing and heart rate, also increases. Weight loss occurs due to taking in less food and possibly burning more energy.1
People taking Phentermine should be monitored while taking the medication and be aware of potential side effects.
20 Side Effects of Phentermine You Should Be Aware Of
Side effects of Phentermine can vary from mild to severe or life-threatening. If you're considering taking Phentermine, understanding the potential adverse health impacts can help you have a conversation with your provider to weigh the risks or benefits.
Common Side Effects:1,3,
- Constipation or diarrhea
- Vomiting or nausea
- Dry mouth
- Hair loss
- Sexual side effects
Serious side effects include:1,3,6
- Seizures or shaking
- Blurry vision
- Uncontrolled muscle movements
- Behavioral changes
- Increased heart rate and blood pressure
- Heart valve disease
- Lung disease (primary pulmonary hypertension)
- Cardiac ischemia
- Addiction and withdrawal symptoms
Impaired judgment and driving or ability to operate machinery are also listed as potential concerns with Phentermine.1
Phentermine Side Effects in Females
Phentermine side effects in females may be even more of a concern, and most of those prescribed the drug are women. The above side effects can impact both men and women, but there may be additional effects in females.
Women's hormones are sensitive to nutrient scarcity or stress. You may think of stress as something external, like a tough commute or a toxic work environment, but physiological stress can be a problem because it means the body must adapt. Sometimes this can be positive, like when we exercise and our muscles adapt. But in the case of certain diet drugs or very low-calorie diets, disruptions in the body caused by stress can impair hormone balance.
Your body needs a certain number of calories for basic but essential functions like breathing or thinking, so if calories drop too low or the body is under stress, it limits the activity of so-called non-essential functions like sex hormone production. As the production of these hormones drops, the menstrual cycle can be disrupted, irregular, or absent.7 Anytime you stop having a period (and aren't pregnant or in menopause), seeing a doctor is a must.
Phentermine can also influence libido and sex drive. For men, erectile dysfunction is a known side effect, but for women, it can affect libido or even cause physical discomfort during sex.1
Since Phentermine is meant to be taken short-term, body weight regain is possible after it is discontinued, especially if diet and exercise habits haven't changed. So not only is it a stimulant with addictive properties that make it harder to quit, but someone taking it may hesitate to stop because they don't want the weight to return.
Phentermine dosage depends on your personal needs and prescription. The dosage can also depend on the specific type of Phentermine you are prescribed.
The typical dosage is 37.5 mg daily, but you should take only what your doctor recommends.1 If you and your doctor decide Phentermine is appropriate, they will prescribe the lowest dose possible.
Phentermine Interactions and Medicines to Avoid
Phentermine can interact with other medications, so it's essential that your healthcare provider knows all prescription drugs and supplements you currently take.
Certain medications should either not be taken or require a closer look if you take Phentermine, including:1
- Monoamine oxidase inhibitors (MAO): These drugs are prescribed for mental health conditions but have many other drug interactions.
- Other weight loss drugs (including herbal products): There isn't enough research on the safety of Phentermine combined with other drugs.
- Selective serotonin reuptake inhibitors (SSRIs): SSRIs are antidepressants like fluoxetine and sertraline and have not been studied in combination with Phentermine.
- Diabetes medications: People taking insulin or other oral drugs for blood sugar management (such as metformin) may need to adjust their dosage if taking Phentermine.
It's also worth noting that it's recommended to avoid alcohol while taking Phentermine because of the potential increased cardiovascular risks.1
When to See a Specialist
Phentermine should always be taken under the guidance of a healthcare professional, and you should always follow medical advice and never take more than prescribed. If misused or without doctor supervision, it could be dangerous.
Contact your healthcare provider immediately if you experience any adverse effects while taking Phentermine. Some common side effects may subside after a few weeks of use, but if you're experiencing heart palpitations, chest pain, swelling in your legs, or if you just don't feel well, it's best to consult your provider immediately.
Learn More About Healthy Ways to Manage Weight Loss with Signos' Expert Advice
The reality is that short-term weight loss medications may not be the solution you're looking for. It can be frustrating to feel like you've tried everything, but the side effects of Phentermine may not be worth the risk—and you deserve to feel good in your body without risking your health.
No magic pill exists to help you reach your goals without addressing diet, exercise, and overall lifestyle. These are the foundational pillars of long-term health, and they cannot be ignored if you want to achieve lasting results. Weight loss medications like Phentermine may provide short-term success, but the weight will likely return without lasting changes to your habits and behaviors.
Weight loss is not a one-size-fits-all solution. Everyone is different, and what works for one person may not work for another. Signos offers a personalized approach to see exactly how your body responds to food, exercise, stress, and more.
Using a continuous glucose monitor (CGM) paired with Signos technology opens the door to analyzing what's happening in your body and making lifestyle adjustments tailored to your needs. Find out if Signos is a good fit for you by taking a quick quiz. You can also learn more about nutrition and healthy habits to help you reach your goals on Signos' blog.
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Topics discussed in this article:
- National Institutes of Health. (n.d.). DailyMed - phentermine hydrochloride capsule. U.S. National Library of Medicine. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=b7fd9015-83f3-431a-90e2-4fe68006754e
- Mark, E. J., Patalas, E. D., Chang, H. T., Evans, R. J., & Kessler, S. C. (1997). Fatal pulmonary hypertension associated with short-term use of fenfluramine and phentermine. The New England journal of medicine, 337(9), 602–606. https://doi.org/10.1056/NEJM199708283370904
- Prescribing information - U.S. Food and Drug Administration. (n.d.). https://www.fda.gov/media/138795/download
- U.S. National Library of Medicine. (n.d.). Phentermine and Topiramate: Medlineplus drug information. MedlinePlus. https://medlineplus.gov/druginfo/meds/a612037.html
- Perez, C. I., Kalyanasundar, B., Moreno, M. G., & Gutierrez, R. (2019). The Triple Combination Phentermine Plus 5-HTP/Carbidopa Leads to Greater Weight Loss, With Fewer Psychomotor Side Effects Than Each Drug Alone. Frontiers in pharmacology, 10, 1327. https://doi.org/10.3389/fphar.2019.01327
- Derosa, G., & Maffioli, P. (2012). Anti-obesity drugs: a review about their effects and their safety. Expert opinion on drug safety, 11(3), 459–471. https://doi.org/10.1517/14740338.2012.675326
- Ryterska, K., Kordek, A., & Załęska, P. (2021). Has Menstruation Disappeared? Functional Hypothalamic Amenorrhea-What Is This Story about?. Nutrients, 13(8), 2827. https://doi.org/10.3390/nu13082827