Phentermine is an appetite suppressant that doctors prescribe in conjunction with exercise and diet adjustments to help patients lose weight. Although phentermine is a commonly used prescription medication for weight loss, there are many myths about it. Believing in various myths creates a situation where people don't know whether the drug could help them or not. Those who use phentermine can have all sorts of doubts about the medication they’re taking, only because misconceptions about the drug are not appropriately addressed. Let’s change that, shall we? Scroll down to see some of the most common myths about phentermine and the real truth.

Myth #1: It’s easy to get phentermine without doctor’s prescription.

Nowadays, a wide range of diet pills is available to overweight and obese people who want to slim down. Some drugs are available in over-the-counter form while others require a doctor’s prescription. Phentermine is a prescription-only medicine meaning one can only obtain it with a prescription issued by their healthcare provider. It is not uncommon for people to come across websites and claims they can get phentermine without doctor’s prescription, but that is simply not the case.

The active ingredient in this drug is phentermine HCI, a chemical compound similar to amphetamines. This particular ingredient is strong and a stimulant which is why the FDA classified it as a controlled substance. For that reason, it is not possible to get “real” phentermine without a prescription. That being said, many manufacturers provide drugs that are similar to phentermine, with a similar name, but without the above-mentioned ingredient. While they may be helpful, those pills are not the “real” phentermine.

Myth #2: Everyone can get phentermine.

Phentermine is not a type of weight loss pill that everyone can take to slim down. Doctors usually prescribe the drug to patients whose BMI is 30 or higher. Phentermine may be prescribed to overweight men and women with BMI 27 in cases when there is at least one weight-related condition such as high cholesterol, type 2 diabetes, hypertension or high blood pressure (1). If your BMI is lower than 27 or even 30, you need to try slim down by adjusting your diet and exercising regularly. Moreover, phentermine is not suitable for men and women with certain health conditions such as hyperthyroidism, coronary heart disease, uncontrolled high blood pressure, and people with a history of drug abuse.

Myth #3: Phentermine alone is enough to slim down.

Actually, this is a common myth regarding weight loss pills in general, and phentermine is not the exception. A commonly accepted belief is that weight loss drugs are enough to slim down without any other lifestyle modifications.

Somehow it is expected that taking pills regularly can lead us to target body weight even if we do not do anything else. That's incorrect, of course.

Phentermine is not meant to work on its own, but it is intended to be used as a part of an overall weight loss plan (2). In other words, phentermine is an adjunct to lifestyle modifications such as doctor-approved exercise and calorie restriction.

The main objective here is to promote weight loss while also helping a patient adopt healthier lifestyle habits that they'll employ even when they stop taking the drug. These lifestyle adjustments lead to successful outcomes and help people maintain their weight loss, i.e., to avoid a common yo-yo effect that comes with unhealthy approaches.

Myth #4: Phentermine has no side effects.

Desire to lose weight and helpfulness of phentermine can make it seem like the drug is entirely safe. But, you need to bear in mind that every medication comes with a certain risk of adverse reactions and phentermine is not the exception.

Some of the most common side effects associated with phentermine are:

  • Dry mouth
  • Vomiting
  • Constipation or diarrhea
  • Unpleasant taste
  • Flushing of the skin

Besides the above-mentioned side effects, phentermine can also induce more serious reactions such as dizziness, restlessness, hypertension, heart palpitations, tremor, insomnia, chest pain, shortness of breath, swelling in legs and ankles, and difficulty to perform exercises that you used to do easily (3).

However, phentermine is well-tolerated by most people. To decrease the risk of adverse reactions, make sure you take the drug according to dosage instructions provided by your doctor. Patients should not increase or decrease dosage on their own.

Myth #5: You can take phentermine as much as you would like.

Not true! Phentermine is not a dietary supplement or one of those diet pills one can get in over-the-counter form. The strong formula is the main reason why phentermine was created primarily for short-term use. That means you cannot really take phentermine as much as you'd like.

Generally speaking, most people take phentermine for three to six weeks. But, the exact length of the treatment depends on how a patient responds to the medication. Do not expect to use phentermine for long-term as it is only intended to act as a short-term weight loss aid. For example, the FDA approval for phentermine is for three months or 12 weeks, and since the drug can be habit-forming it's clear why patients can use phentermine for up to 12 weeks maximum, but in many cases, the course of the treatment is shorter than that.

That being said if a doctor believes that a patient may benefit from a longer treatment, they may evaluate the possibility of prescribing more rounds of phentermine, but with at least one-month-long breaks in between the cycles.

Myth #6: Women can’t use birth control when taking phentermine.

Some drugs reduce the efficacy of weight loss pills, and drug interactions happen, so it's not so unusual for women to assume that they should not use their birth control pills when they are taking phentermine. If you can conceive, it's important to use contraception while you're taking this weight loss drug. In fact, phentermine alone hasn't been shown to affect the effectiveness of birth control pills. That being said, the drug has been associated with irregular vaginal bleeding in some cases, especially when taken in combination with topiramate. The occasional occurrence of bleeding (in some cases) does not undermine the efficacy of birth control pills in women who take phentermine (4). Make sure you consult your doctor about this subject. Before you start using phentermine, feel free to address the birth control pills subject to see what your doctor recommends.

Myth #7: Nothing will happen if I take phentermine when pregnant.

You may get the urge or desire to take phentermine during pregnancy in a bid to avoid gaining excessive weight. That's not a good idea, and your doctor will not issue a prescription for this type of use. Phentermine is not recommended during pregnancy because weight loss is not beneficial to pregnant women and may lead to fetal harm (5). Although evidence on this subject is scarce, it is not advised to take this drug when pregnant. The same goes out to mothers who breastfeed. It is not known whether phentermine reaches to human milk or not, but other types of amphetamines are known to reach the breast milk. The potential risk of increased harm to the infant is the reason why you should not use this drug when breastfeeding.

Myth #8: Phentermine doesn't really work; it's just a marketing ploy so the manufacturer could increase sales.

How many times have you thought that phentermine wasn’t effective or that all the benefits of this drug were just a marketing strategy? It’s not unusual to be skeptic about the whole thing, but a growing body of evidence confirms that phentermine indeed works.

For example, a study from the Yonsei Medical Journal found that using phentermine for a short period of time caused a significant weight reduction and decreased the belly fat without affecting the health of the participants in the study (6). Studies show that the expected average weight loss with phentermine is 5% of the initial body weight, but after 12 weeks of regular use, it can be as high as 10%. In other words, a patient weighing 200lbs (90.7kg) can lose 10 to 20lbs or 4.5 to 9kg, thanks to this drug (7).

When talking about the efficacy of phentermine, it is important to mention that a study from the journal Obesity found long-term treatment leads to greater weight loss (8). Of course, this doesn't mean you should use it for longer than three months. Remember, your doctor may prescribe the drug again only when a patient can really benefit from it.

Myth #9: Some people gain weight when they stop taking phentermine.

Yo-yo effect where a person regains the lost weight and puts on even more pounds is a common occurrence for many overweight or obese men and women. This unwanted scenario typically happens when a person employs an unhealthy weight loss regimen. You have probably heard or read claims that people gain weight once they stop using phentermine. The reality is entirely different, though.

With phentermine, you don't have to worry about regaining your weight. Why? It's simple; phentermine is just an adjunct to a low-calorie diet and regular exercise, and once you adopt those healthy habits, you will be able to maintain weight loss even when not using phentermine. In other words, if you stick to the program and listen to your doctor's recommendations, then you don't have to worry about unfavorable outcomes.

Myth #10: People who take phentermine are positive on a drug test.

Phentermine is chemically related to amphetamine, so it may be a source for concern among people who feel their drug tests will show positive. That's not how it works, actually. Bearing in mind that phentermine is similar to amphetamine, it can produce a "false positive" result on the urinalysis test. Basically, the urinalysis may show that you're positive after which a confirmatory test is ordered. This test shows that the substance present in your urine was phentermine, not methamphetamine, and amphetamine. That's when the initial result is ruled as the above-mentioned "false positive." The best thing to do is that, if you need to take the drug test, you disclose this information to the testing lab. Mention that you're taking phentermine, and that's it. But, as you can see, you don't have to worry about it as the confirmatory drug test will show the negative outcome.

Myth #11: Phentermine can get me high.

Actually, not really! People who use phentermine do not report that the drug gets them high. After all, phentermine is a stimulant and appetite suppressant that works to promote a feeling of fullness and prevent you from overeating. When taking doses recommended by the doctor, you don't have to be afraid of any psychoactive effects. Some people use large doses to achieve a high effect, but these can be quite dangerous and lead to overdose. The bottom line is that phentermine can’t really get you high and you should always stick to the dosage recommended by your doctor.


Phentermine is one of the most widely used weight loss pills, yet there are many myths and misconceptions about it. Unfortunately, these misconceptions may present phentermine as some dangerous drug, although that is not the case. As seen throughout this post, phentermine is effective, safe, and poses as a valuable addition to a weight loss plan. Always consult your doctor about all the questions and concerns you have about phentermine.



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