What is Phentermine?

Published on July 12, 2024
What is Phentermine

Phentermine, think of it as your secret weapon to drop weight, but catch is, you need a doc’s prescription to grab it. Way back in 1959, the FDA folks said it was cool for anyone 16 or older to use, but only let you have it for a quick 12 weeks. (1)

Phentermine was used in conjunction with a variety of weight loss drugs that hit the market in the 1990s. One popular combination in the 90s was phentermine and fenfluramine. Due to reports of potentially fatal heart problems, the FDA recalled the treatment’s primary medications, fenfluramine, and dexfenfluramine. (1)

Phentermine itself continued to be one of the most prescribed drugs in the world due to its relative safety and efficacy.

Phentermine is sold under brand names including Adipex-P, Lomaira, and Suprenza. Qsymia combines the weight loss drugs phentermine and topiramate into one convenient tablet. (1)

Phentermine is a stimulant and appetite suppressant that requires a prescription in order to purchase. However, some unscrupulous online pharmacies allow its distribution even without a doctor’s consent, which of course, is never without risks.

Because it has the potential for drug abuse and dependence, phentermine is classified as a Schedule IV drug. . The structure of these substances is very similar to that of amphetamine, a banned stimulant. (1)

If your doctor has determined that you continue to suffer from obesity (a BMI of 30 or more) despite improving your diet and exercise, which remains to be the first line of treatment for obesity, (1) you may be given phentermine. Alternatively, if you have a BMI of 27 or higher and at least one weight-related health problem, such as hypertension, dyslipidemia, or type 2 diabetes.

Additional Information About Phentermine

An anorectic substance known as phentermine was first released as part of a combination medication used to treat obesity. Often referred to as an “atypical amphetamine,” it shares chemical similarities with amphetamine. Phentermine is classed as a Schedule IV substance because there is no evidence suggesting that it is addictive, but the actual potential is low. (1)

By 1960, the drug was extensively utilized. Several reports of aberrant valves in over 30% of the users led to the discontinuation of the original medication, which was developed by combining phentermine with fenfluramine and dexfenfluramine. Later, in 2012, phentermine was approved both by itself and in conjunction with topiramate as a novel option that could achieve the same results with smaller doses of phentermine. (1)

The term “exogenous obesity” is used to describe cases of becoming overweight due to eating more calories than the body needs. Increased fat storage is a common symptom of this illness. Over two-thirds of American adults are overweight or obese, and one-third of the population is obese. The global prevalence of obesity has almost doubled in recent decades. (1)

Phentermine with or without topiramate may be helpful as a short-term adjunct, not exceeding a few weeks, for the management of exogenous obesity in patients with an initial BMI of 30 kg/m2 or more than 27 kg/m2 who also have other risk factors like diabetes, high blood pressure, or hyperlipidemia. (1)

Although an increase in leptin production has been suggested as phentermine’s primary mechanism of action, it is generally agreed that additional processes must also be at play. Some research suggests that the increased energy expended at rest is responsible for the weight reduction impact. (1)

Studies comparing phentermine to a placebo suggest that it is more effective at helping people lose weight than the placebo when used alone or in combination. After stopping phentermine therapy, patients were likely to keep the weight off. In addition, while being a relative of amphetamines, it has not been approved to cause the same stimulant effects on the neurological system, increased heart rate, accelerated breathing, or prolonged QT interval seen with amphetamines. (1)

Mechanism of Action

Phentermine is among the drugs called sympathomimetic amines, although it works indirectly by stimulating the release of noradrenaline from presynaptic vesicles in the lateral hypothalamus. When the concentration of noradrenaline in the synaptic cleft rises, beta2-adrenergic receptors become activated. (1)

Since phentermine increases levels of norepinephrine and dopamine and indirectly affects serotonin, it is characterized as an indirect sympathomimetic. It has been suggested that phentermine blocks the neuropeptide Y signaling pathway, which is crucial in stimulating appetite. (1)

As a result of these factors working together, the body is in a constant state of “fight or flight,” which suppresses the hunger response since the focus is on meeting the body’s most pressing energy needs.

Finally, phentermine may act as a mild inhibitor of monoamine oxidase, according to certain studies. However, it was never labeled as one of the monoamine oxidase inhibitors because this mechanism rarely results in a noticeable clinical effect. (1)

The pharmacokinetics of phentermine is dose-dependent. Maximal concentration was reached 6 hours after oral administration of 15 mg, and its bioavailability was unaffected by high-fat meals. In clinical trials, steady-state plasma concentrations were reported to be close to 200 ng/ml. (1)

Minimal p-hydroxylation, N-oxidation, N-hydroxylation, and conjugation occur in phentermine. Approximately 5% of the dose is converted into N-oxidized and N-Hydroxylated metabolites, whereas the overall amount of medication metabolized is only about 6%. Seventy percent to eighty percent of an oral dose of phentermine is recovered unchanged in the urine. (1)

How Does Phentermine Aid Weight Loss?

Although phentermine is typically prescribed for weight loss, it has also been studied with the anticonvulsant topiramate for treating binge eating disorders and other eating disorders (BED). Several human investigations have established that phentermine can aid in significant weight loss. (2)

Although the Food and Drug Administration (FDA) only licensed phentermine for use for up to 12 weeks, doctors frequently prescribe it off-label for more extended periods. If your doctor recommends a break from the medicine for a particular time, you may need to take that time off before starting the prescription again. (2)

The problem is that not everyone has the same reaction to this drug. After three months of treatment, your doctor may suggest you stop taking the medication if you haven’t seen significant weight loss. (2)

In addition, it seems to lose its efficacy over time, especially after more than a year of use.

Phentermine has been demonstrated to help with weight loss, but it may be even more successful when coupled with topiramate. (2)

Topiramate, like phentermine, is a medication that can suppress appetite while simultaneously being used to treat seizures. People who took a drug with phentermine and topiramate dropped an average of 16 pounds (7.3 kg) compared to the placebo group. (2)

It rose to 17.8 pounds for those who took the drug for longer than 56 weeks (8 kg). Users who took phentermine-topiramate improved insulin sensitivity, blood sugar management, and blood pressure, in addition to decreased waist circumference. (2)

Users are taking phentermine-topiramate saw improvements in insulin sensitivity, blood sugar management, and blood pressure in addition to a decrease in waist circumference. (2)

It Helps Treat Some Forms of Eating Disorders

Individuals with BED and bulimia nervosa may benefit from the combination of phentermine and topiramate in lowering binge eating episodes. (3)

To the point of discomfort, extreme overeating is a hallmark of binge eating disorder (BED). It is also linked to emotions of helplessness and guilt after that. (3)

People with bulimia nervosa binge eat like those with BED, but they also engage in compensatory behaviors like purging to undo the damage done by the binge. (3)

Drug combinations of phentermine and topiramate have been shown to significantly reduce binge eating episode frequency in a small 12-week study in overweight persons with BED. (3)

In further 12-week research, participants with BED or bulimia nervosa were given either the medication combination or a placebo. (3)

Binge eating days dropped from 16.2 to 4.2 while on phentermine and topiramate for 28 days. Those receiving a placebo did not experience the same improvements. (3)

While these findings show promise, more extensive research is required. And don’t forget to talk to a doctor if you suspect you have BED, bulimia nervosa, or another eating disorder. (3)

Are There Any Side Effects?

No drug is without adverse effects or possible drug interactions, especially when used incorrectly. Most are mild including constipation, dry mouth, increased heart rate, nervousness, tingling sensation in hands or feet, and sleeplessness.

Some more serious side effects may include increased blood pressure and primary pulmonary hypertension, which can be controlled easily with the help of a professional.


Phentermine, a diet drug that requires a doctor’s prescription yet is effective in promoting weight loss, is one such medication. It was common practice in the 1990s to combine phentermine with other weight loss medications. (3)

Phentermine dosages are not standard and might change based on the formulation and strength of the drug. Phentermine only came in 15, 30, and 37.5 mg levels until 2016. If your doctor has ordered a more significant dose for you (15, 30, or 37.5 mg), take it once daily, preferably before or two to three hours after breakfast. Avoid taking your last dose too close to bedtime to avoid insomnia, trouble sleeping, or staying asleep. (3)


  1. https://journals.sagepub.com/doi/10.1177/2042018820918789
  2. https://medlineplus.gov/druginfo/meds/a682187.html
  3. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2687747/