Obesity is certainly one of the most common, severe, and costly health problems today. The latest data and statistics suggest that more than 93 million adults in the US are obese (1). Its prevalence in kids and adolescents are also alarmingly increasing (2). Even more alarming is that the causes of premature deaths, including heart disease, stroke, type 2 DM, and cancer, are related to obesity (1).

While eating a balanced diet and getting physically fit are two of the most effective ways to control obesity and decrease your risks of developing chronic disease, many find it difficult to sustain such lifestyle changes; hence they get easily impressed with the well-advertised effects of weight loss drugs like Phentermine. However, not everyone is allowed to take Phentermine for weight loss. There are several reasons why one can’t use Phentermine; most of which are health-related and may put your life and health in great danger.

In this article, we’ll discuss the basics of Phentermine use, its possible side effects, and the most common reasons why your doctor may refuse to add Phentermine on your weight loss regimen. Read on to know more.

What is Phentermine?

Phentermine was first marketed in 1959. It is a weight loss drug commonly prescribed by doctors to treat patients suffering from severe obesity. You can get it in different brand names like Adipex-P, Lonamin, Pro-Fast, and Qsymia. Known as an effective anti-suppressant, it can help reduce your cravings and binge-eating episodes. It stimulates encourages the production and release of more dopamine and norepinephrine, which can both affect your appetite (3). Some also say that it affects the way your body utilizes energy to help you burn more calories.

It has been proven to work well with exercise, low-calorie diet, and behavioral changes. Phentermine is usually prescribed for a short time only, approximately 3-6 weeks (4). Phentermine is available in four forms – capsule, tablets, disintegrating tablets, and extended-release capsules. Each one of this form has its own dosing instructions, which you can find the packaging labels.

Though it is generally safe when taken correctly, others may still not qualify to use Phentermine.

15 Reasons Why May Not Be Advised to Use Phentermine

1. You Are Not Overweight or Obese

As we have mentioned repeatedly, Phentermine is commonly used for weight loss and the treatment of obesity. This drug is highly recommended for patients with an initial body mass index (BMI) of ≥30kg/m2 or ≥27kg/m2 in the presence of other weight-related problems (5).

BMI is a measure of body fat in one’s body based on his or her height and weight. We calculate BMI to know if we weigh healthily or not. The ideal BMI for most adults ranges from 18.5 to 24.9. Generally, individuals who have a BMI lower than 18.6 are underweight, while those who have a BMI of 25 to 29.9 are considered overweight. If you exceed 30, you are already obese (6).

Therefore, if you are not overweight or obese and are not suffering from an eating disorder, your doctor may refuse to give you a prescription for Phentermine.

2. You Are Hypertensive and Experience Heart Palpitations

Approximately 75 million people or 32% of the US population are diagnosed with high blood pressure or hypertension; that is 1 in 3 adults (7). It increases one’s risk of developing cardiovascular diseases. What many people don’t know is that Phentermine, as a stimulant, can cause a serious increase in your blood pressure by constricting the peripheral blood vessels, making your heart work harder. This can cause irregular, strong, and rapid heartbeats known as palpitations (8).

While others may still allow you to use low doses of Phentermine if your BP is well controlled and your palpitations are just mild, most doctors will not take the risk and opt not to prescribe Phentermine for your own good and safety.

3. You Have Glaucoma and Other Visual Problems

Glaucoma is a condition that involves damage to the optic nerve, which serves an important function to keep a good vision, caused by abnormally high pressure in the eye. Evidence suggests that glaucoma is the second leading cause of blindness in the world, and about three million Americans have it (9).

Although it can occur at any age, glaucoma is more common among older adults. Symptoms of glaucoma may vary depending on the type, but they would normally include patchy blind spots in the side and central vision, tunnel vision during advanced stages, headache, pain, blurred vision, redness, and many others.

Phentermine users may complain of blurring or doubling of vision due to excessive sympathetic activation, which includes dilatation of the pupils, but this effect is only temporary (10). However, this increase in intraocular pressure may aggravate the condition of patients with glaucoma. While reducing the dosage may be helpful, most doctors think it is safer to not use Phentermine until glaucoma is treated.

4. You Have an Overactive Thyroid

Your thyroid, a butterfly-shaped gland, plays an important role in several processes in the body. However, it is susceptible to several health problems of which hypothyroidism and hyperthyroidism are the most common. Hypothyroidism refers to an underactive thyroid gland marked by a decreased production of hormones whereas the latter is an overactive gland producing excessive amounts of thyroid hormones.

Evidence suggests that hyperthyroidism is ten times more common in women than in men (11). It affects nearly 2-5% of all women, ages 20 to 40; that is 1 to 2 cases per 1000 people per year. While it is usually caused by a family history of thyroid problems and an autoimmune condition known as Grave’s disease, medications like Phentermine can also increase the gland’s activity; hence many doctors do not want to prescribe this medication to hyperthyroid patients for weight loss. The symptoms of hypothyroidism are many, and they may be aggravated by Phentermine, which will greatly affect the quality of your life.

5. You Are Planning to Have A Baby

Evidence suggests that users of Phentermine may experience changes in sexual desire. While there have been reported cases of a significant increase in libido brought about by the induced catecholamine release, others experience a reduction in sexual drive or impotence. This can be attributed to stress and anxiety or the excessive sympathetic reaction, which interferes with a man’s ability to keep an erection. What’s more alarming are the reports saying that Phentermine can cause a temporary shrinkage of the genitals (10).

While getting enough rest and sleep may help limit stress, and tweaking doses may help correct sexual dysfunction, doctors prefer not to prescribe Phentermine to people who are having trouble conceiving a baby.

6. You Are on MAO Medications

Monoamine oxidase (MAO) inhibitors are the very first antidepressant drugs ever formulated. They act on neurotransmitters, changing the brain’s chemistry to help ease the symptoms of depression. Normally, an enzyme known as monoamine oxidase removes neurotransmitters like dopamine, serotonin, and norepinephrine from the brain. The role of these drugs is to prevent this from happening (12).

If you are experiencing depression and have been given MAO inhibitors in the last 2 weeks, your doctor may not allow you to take Phentermine. Why? Both MAO inhibitors and Phentermine are CNS stimulants. Phentermine may also inhibit the activity of monoamine oxidase (13), which will significantly increase the effects of the anti-depressant at a dangerous level. Complications like damage to the heart valves, pulmonary arterial hypertension, and an increase in BP may occur (14).

7. You Have Problems Sleeping

Phentermine is a central nervous system stimulant and those who are taking this drug, especially at higher doses, may experience insomnia. It likewise disrupts the normal circadian rhythm and causes anxiety or stress, which can also prevent a person from getting a good night’s sleep (10).

The duration of the action of Phentermine after administration may range from 4-14 hours, depending on the dosage (15). While this side effect may be contingent upon the time of administration, doctors still prefer not to prescribe Phentermine to patients with severe sleeping problems.

8. You Have A History of Drug Abuse

Phentermine as a strong stimulant can be very addictive; hence it is only recommended for short-term use. Taking Phentermine for 6 months or longer may lead to psychological dependence, insomnia, restlessness, increased heart rate, and more (10).

Phentermine may cause habit-forming effects, which is dangerous to people who already have a history of drug abuse. For that reason, your doctor may not prescribe you this drug as it might do more harm than good in the long run.

9. You Are Pregnant or Breastfeeding

A lot of women gain weight during pregnancy, and that is normal. However, some who gain a lot more than expected are struggling to slim down. This tempts them to try using weight loss medications like Phentermine.

However, your doctor will definitely not allow you to take Phentermine while you are still pregnant or even after delivery, especially if you plan to breastfeed your baby. Studies have shown evidence of fetal abnormalities and harmful infant effects (16). Your doctor may suggest other techniques to keep your weight in a healthy range or to slim down.

10. You’ve Been On Phentermine for Quite Some Time

As mentioned earlier, Phentermine is meant to be used only for a short period of time. Using this drug for more than 12 weeks does not only increase your risk of addiction, you are more likely to gain your lost weight back too!

So, your doctor may stop prescribing Phentermine to you if you have already reached the recommended duration of treatment. You will have to take a break, a couple of weeks or months maybe before you can start using Phentermine again. In some cases, a switch to other drugs or weight loss programs may be necessary.

11. You Have Allergies

Allergies to Phentermine are uncommon, but not impossible. This is commonly due to an interaction with another drug. You may experience skin redness, itchiness, swelling, shortness of breath, and vomiting (10).

Any history and sign of allergic reactions should be taken seriously because most can cause life-threatening events. Doctors will not risk giving you Phentermine if you have any type of allergy just to lose weight. You will most likely be given other tips to slim down.

12. You Are A Minor or An Elderly

Phentermine is not recommended for kids age 16 and below because the safety of this drug in this age group is not yet established (17). Also, this is not commonly given to elderly patients as they are more prone to heart, liver, and kidney problems (16). So, if you are too young or too old to worry about slimming down, your doctor may restrict you from using Phentermine and recommend other techniques instead.

13. You Are Suffering From Depression and Anxiety

Due to excessive sympathetic effects, prescribing Phentermine to patients with depression and anxiety might not be the best idea. Phentermine will alter blood flow, neural activity, and the production of hormones and neurotransmitters. This leaves you in a constant state of “fight-or-flight”, accompanied by irritability and anger (10). This will only make the depression and anxiety problems worse, possibly endangering your life.

14. You Are An Athlete

Some users of Phentermine complain about muscle cramps, which are commonly experienced in the back, legs, and arms. There are several ways by which this drug can cause cramps, but the most popular culprits include electrolyte depletion, dehydration, and excessive muscle tension (10).

While drinking a lot of water, getting a good supply of electrolytes, and rest can help, your doctor may refuse to prescribe you with Phentermine especially if the benefits will not outweigh the risks involved and will greatly affect your performance and career.

15. You Drive Regularly

Dizziness is yet another common complaint of Phentermine users. The exact mechanism as to how this drug causes dizziness is not well established yet, but experts are relating it to the increased sympathetic nervous activation. There is constriction of the blood vessels, affecting blood flow to the brain (10).

Phentermine also offers a strong psychostimulatory effect that some users end up having “shakes” or tremors. There is a drastic increase in the levels of catecholamine, which prompts the body to mobilize energy stores and be highly stimulated. Tremors most often occur among users who are taking high doses (10).

While decreasing the dosage may help control these side effects, your doctor will most likely refuse to give you Phentermine to keep you safe, especially when you are always out on the road.

Key Takeaway

Phentermine is an excellent weight-loss drug that, when combined with a healthy diet and regular exercise, can easily bring you significant results. However, it is not suitable for everybody. Depending on your health status and daily activities or routines, your doctor might decide against using Phentermine to help you slim down. After all, there are several other effective and safer ways to lose weight and not put your life in great danger.


(1) https://www.cdc.gov/obesity/data/adult.html
(2) https://www.cdc.gov/obesity/data/childhood.html
(3) https://www.drugs.com/tips/phentermine-patient-tips
(4) https://www.livestrong.com/article/544387-what-does-phentermine-do-to-the-body/
(5) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4011567/
(6) https://www.nhlbi.nih.gov/health/educational/lose_wt/BMI/bmicalc.htm
(7) https://www.cdc.gov/dhdsp/data_statistics/fact_sheets/fs_bloodpressure.htm
(8) https://www.healthline.com/health/phentermine-oral-capsule#other-warnings
(9) https://www.glaucoma.org/glaucoma/glaucoma-facts-and-stats.php
(10) https://mentalhealthdaily.com/2016/05/24/phentermine-side-effects-adverse-reactions-list/
(11) https://www.myvmc.com/diseases/hyperthyroidism/
(12) https://www.mayoclinic.org/diseases-conditions/depression/in-depth/maois/art-20043992
(13) https://www.ncbi.nlm.nih.gov/pubmed/10799660
(14) https://infophentermine.com/interaction-mao-inhibitors-phentermine/
(15) https://www.empowerpharmacy.com/drugs/phentermine-hcl-capsules.html
(16) https://www.mayoclinic.org/drugs-supplements/phentermine-oral-route/before-using/drg-20075169
(17) https://www.healio.com/endocrinology/obesity/news/online/%7Bd8952019-0e73-4383-b2d5-48f859fb4ece%7D/fda-approves-low-dose-phentermine-for-obesity

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