Phentermine

In addition to diet and exercise, phentermine suppresses hunger and may help a person lose weight. However, some people may not be able to use the drug safely. Although regular exercise and a well-balanced, low-calorie diet are the mainstays of weight management, several medications can offer potent assistance.

Phentermine, one of the most widely used medications for weight loss worldwide, is one such medication.

When combined with exercise and a lower-calorie diet, it is beneficial for temporary weight loss.

However, there are risks and adverse effects associated with using phentermine for weight loss.

What is Phentermine?

Phentermine helps people who are overweight lose weight more quickly. It is used to treat obesity, particularly in those with risk factors including diabetes, high blood pressure, or high cholesterol, in addition to diet and exercise.

Amphetamines and phentermine are comparable. It boosts blood pressure and pulse rate while decreasing appetite by stimulating the brain and central nervous system.

The class of drugs known as anorectics includes phentermine. It functions by decreasing your hunger.

Phentermine is a substituted phenethylamine and amphetamine that is also referred to as α,α-dimethylphenethylamine or α-methylamphetamine. It is an amphetamine derivative where the alpha carbon has a second methyl group. Methamphetamine (N-methylamphetamine) and other methylamphetamines, including 4-methylamphetamine, are positional isomers of the substance.

There are several other forms of phentermine, such as pentorex, cericlamine, cloforex, clortermine, etolorex, mephentermine, 3,4-methylenedioxyphentermine (MDPH), and 3,4-methylenedioxy-N-methylphentermine (MDMP or MDMPH). Like phentermine, some of these medications—such as mephentermine, cloforex, chlorphentermine, and clortermine—have been sold as pharmaceuticals that decrease appetite.

What is the mechanism?

Phentermine is a member of the class of medications known as anorectics, or appetite suppressants.

By suppressing your appetite, phentermine helps you consume fewer calories. This may eventually result in weight loss.

Phentermine is believed to work by raising neurotransmitter levels in the brain, while the precise mechanism by which it suppresses appetite is still unknown.

Your body uses neurotransmitters, which include the chemicals dopamine, serotonin, and norepinephrine, as chemical messengers.

Your sense of hunger diminishes as your levels of these three hormones rise.

Within a few weeks, though, you can develop a resistance to phentermine’s appetite-suppressing effects. Consult the medical practitioner who prescribed it if that occurs.

For what purpose is it used?

Although phentermine is most frequently used to assist people in losing weight, it can also be used in conjunction with topiramate to treat specific eating disorders, like binge eating disorder (BED).

Beneficial for losing weight

Phentermine has been proven in numerous clinical investigations to promote significant weight loss.

Healthcare providers frequently prescribe phentermine off-label for longer usage, even though the FDA only approved it for short-term use of fewer than 12 weeks.

Doctors may prescribe it intermittently, which means you stop taking it for a predetermined amount of time and then start taking it again.

Phentermine users often lose 3% of their starting body weight after three months and 5-7% after six months. Some claim to have lost considerably more weight than this.

To put this into perspective, a person weighing 200 pounds (90.7 kg) would lose 10 to 14 pounds (4.5 to 6.4 kg) if they lost 5 to 7% of their body weight.

But not everyone reacts to this drug in the same manner. A medical expert might advise stopping the medicine if, after three months of use, you haven’t lost much weight.

Furthermore, after more than a year of continuous use, its efficacy seems to decline progressively.

Phentermine is useful for losing weight, however, it might be more effective when used with topiramate.

Similar to phentermine, topiramate has been used alone to treat seizures and also has appetite-reducing effects.

Qsymia is a brand name for a combination drug that contains both topiramate and phentermine. As a supplement to healthy lifestyle practices, including increased exercise and a diet low in calories, the FDA authorized it in 2012.

Phentermine and topiramate together were linked to the highest chances of losing at least 5% of starting body weight when compared to three other frequently prescribed medications for weight loss.

Furthermore, individuals who took a medicine that contained phentermine and topiramate lost an average of 16 pounds (7.3 kg) when compared to those who took a placebo, according to a recent review of research.

This figure rose to 17.8 pounds (8 kg) for patients who took the drug for longer than 56 weeks.

Additionally, the dose increased the amount of weight loss. On average, the following weight was lost:

For a 3.75/23 mg dose of phentermine/topiramate, weigh 7.8 pounds (3.55 kg); for a 7.5/46 mg dose, weigh 16.27 kg; and for a 15/92 mg dose, weigh 18.25 kg.
Weight loss among phentermine-topiramate users resulted in a notable reduction in waist circumference, along with enhanced insulin sensitivity, better blood sugar regulation, and a drop in blood pressure.

Potentially beneficial for specific eating disorders

People with bulimia nervosa and BED may have less binge eating if they take topiramate and phentermine together.

Eating a lot of food, frequently, rapidly, and to the point of pain, is a hallmark of BED. Additionally, it is linked to feelings of guilt or shame following the binge, as well as a sense of losing control during the binge.

The binge-eating behavior associated with BED is also present in bulimia nervosa, but self-induced vomiting and other compensatory behaviors are also present.

Taking phentermine and topiramate together significantly decreased the frequency of binge-eating episodes in a small 12-week study of overweight BED patients.

Individuals with bulimia nervosa or BED were given either a placebo or the medication combination in a different 12-week research.

The number of binge-eating days among the individuals dropped from 16.2 to 4.2 during 28 days of therapy with the phentermine and topiramate combination. The placebo group did not experience the same outcomes.

Even though these findings are encouraging, more extensive research is still required.

It’s important to consult a healthcare provider if you believe you have a disordered eating disorder, bulimia nervosa, or BED.

Medical applications

In addition to exercise and calorie restriction, phentermine is used temporarily to aid in weight loss when these methods are insufficient.

The majority of weight reduction happens in the first few weeks of using phentermine, which is authorized for usage for up to 12 weeks. Significant loss, however, persists into the sixth month and has been demonstrated to continue until the ninth month at a slower pace.

Contraindications

It is not advised to use it with monoamine oxidase inhibitors (MAOIs) or selective serotonin reuptake inhibitors (SSRIs) or during pregnancy or lactation.

Phentermine should not be used by anyone who:

Have a history of substance abuse.
Are on monoamine oxidase inhibitors (MAOIs) or have taken one in the previous 14 days; have glaucoma, hyperthyroidism, or cardiovascular illness; or are allergic to sympathomimetic amine medications.
Are pregnant, planning to conceive, or breastfeeding.

Before taking this drug.

Phentermine should not be used if you have an allergy to it or if you have:

A history of heart disease, including stroke, congestive heart failure, cardiac rhythm issues, and coronary artery disease;
extreme or unmanageable hypertension;
Hyperactive thyroid;
glaucoma
excessive anxiousness or agitation;
a history of substance misuse; or
if you use other diet medications.

If an MAO inhibitor has been taken within the last 14 days, avoid taking phentermine. There may be a harmful medication interaction. Isocarboxazid, linezolid, methylene blue injection, phenelzine, rasagiline, selegiline, tranylcypromine, and other medications are examples of MAO inhibitors.

Even if you are overweight, losing weight during pregnancy might have negative effects on the fetus. If you are pregnant, avoid using phentermine. If you become pregnant while undergoing treatment, notify your doctor immediately.

It is not recommended to breastfeed while taking this medicine.

Tell your doctor if you have any of the following to ensure that this medication is safe for you:

coronary artery disease or heart disease;
an issue with the cardiac valves;
increased blood pressure;
diabetes (you may need to modify the dosage of your diabetes medication); or
renal disease.

Anyone under the age of sixteen is not permitted to use phentermine.

How is phentermine to be taken?

As directed by your physician, take phentermine precisely as directed. Read all drug instructions or instruction sheets, and do as directed on your prescription label. Sometimes, your doctor will adjust your dosage.

Typically, doses are taken either one to two hours after breakfast or before. Follow your doctor’s medication instructions to the letter.

Never take more phentermine than is recommended or for longer than is advised. Increasing the dosage of this medication won’t increase its effectiveness and may have major, maybe fatal, negative effects.

This medication should only be used temporarily. After a few weeks, the effects of appetite suppression might subside.

Phentermine could lead to habit formation. Abuse can result in overdose, addiction, or even death. It is illegal to sell or give away this medication.

If you feel that this medication is not functioning as effectively or if you have not dropped at least 4 pounds in 4 weeks, give your doctor a call right away.

Avoid abruptly stopping this medication since this may cause unpleasant withdrawal symptoms. Find out from your doctor how to discontinue using this medication safely.

Keep out of the heat and moisture and store at room temperature. When not in use, keep the bottle tightly closed.

Dosing information

Normal Adult Obesity Dosage:

8 mg taken three times daily half an hour before meals, or 15–37.5 mg taken once daily either before or two hours after breakfast.

Use: To treat exogenous obesity in patients with an initial body mass index (BMI) of 30 kg/m2 or higher, or a BMI of 27 kg/m2 or higher in the presence of other risk factors (e.g., controlled hypertension, diabetes, hyperlipidemia), this short-term (a few weeks) adjunct is used in conjunction with a weight-loss program based on exercise, behavioral modification, and calorie restriction.

Normal Dosage for Obesity in Children:

For those aged 17 and up, take 8 mg three times a day, half an hour before meals, OR

Once daily, take 15–37.5 mg orally either before or two hours after breakfast.

What occurs if I don’t take my dose?

If it is late in the day, skip the missed dose and take the medication as soon as possible. Avoid taking two doses at once.

What would happen if I took too much?

Get help right away or contact the Poison Help Line at 1-800-222-1222. Phentermine overdoses can be lethal.

Confusion, panic, hallucinations, excessive restlessness, nausea, vomiting, diarrhea, cramping in the stomach, feeling depressed or exhausted, irregular heartbeats, weak pulse, seizures, or sluggish breathing (perhaps stopping breathing) are all signs of an overdose.

What to avoid?

Until you know how phentermine may impact you, stay away from driving and other dangerous activities. You might not react as well.

Side effects may occur if alcohol is consumed while taking this medication.

Side effects of phentermine

If you have any of the following symptoms of a phentermine adverse reaction: hives, trouble breathing, or swelling of your face, lips, tongue, or throat, get emergency medical attention.

Serious adverse effects are possible with phentermine. Make an immediate call to your physician if you have:

experiencing dyspnea, even with minimal effort;
chest pain, a sense of immediate fainting;
swelling of the feet or ankles;
heart palpitations or fluttering in the chest;
shivering restlessness and difficulty falling asleep;
abnormal behavioral or emotional shifts; or
Severe headache, blurred vision, ear or neck pounding, anxiety, and nosebleeds are all signs of increased blood pressure.

Common phentermine side effects may include:

Itching,
headache,
nausea,
dry mouth,
bad taste,
diarrhea,
constipation,
stomach pain,
or a rise or fall in desire for sex.

Which medications will affect phentermine?

Pulmonary hypertension is a rare and deadly lung condition that can occur when phentermine is taken with other diet pills like fenfluramine (Phen-Fen) or dexfenfluramine (Redux). Without your doctor’s approval, avoid taking phentermine with any other diet pills.

Phentermine can interact with a lot of medications. This covers vitamins, herbal goods, and both prescription and over-the-counter medications. This list does not include every conceivable interaction. Inform your doctor of all the medications you are taking now, as well as any that you decide to start or stop taking.

Conclusion

The appetite suppressant and weight loss medication phentermine is only available with a prescription and is only permitted for short-term use. It is authorized for long-term use, and the combination of phentermine and topiramate seems to be even more beneficial than phentermine alone.

Additionally, it might lessen bingeing episodes in BED sufferers. Constipation, weariness, irritability, dry mouth, and dizziness are typical adverse effects. If you have severe kidney or liver illness, glaucoma, hyperthyroidism, heart disease, or are pregnant or nursing, you should not use this drug.

Additionally, if you’re taking any drugs, including monoamine oxidase inhibitors (MAOIs), make sure to speak with a healthcare provider before using phentermine. Despite its potential as a weight-loss aid, phentermine is not advised as a first-line treatment.

FAQs

What is the purpose of phentermine oral capsules?

Phentermine oral capsules are only used to treat obesity temporarily (a few weeks). For those who are overweight and extremely obese and have certain health risk factors, this medication helps them lose weight. Among these risk factors include diabetes, high blood pressure, and high cholesterol.

For what duration is phentermine used?

For a brief time, phentermine was used to help people lose weight, both in addition to and in place of exercise and calorie restriction, if these methods were insufficient. The majority of weight reduction happens during the first few weeks of phentermine use, which is permitted for up to 12 weeks.

Is phentermine a substance under control?

However, it is categorized as a prohibited substance in many nations because of its similarity to amphetamine. According to the Convention on Psychotropic Substances, phentermine is classified as a schedule IV substance internationally. According to the Controlled Substances Act, it is categorized as a Schedule IV controlled substance in the US.

Does phentermine require a prescription to be taken?

The medication phentermine is prescribed. It is available as an oral disintegrating tablet, an oral capsule, and an oral tablet. Adipex-P is a brand-name medication that contains phentermine oral capsules. It is also offered as a generic medication. Generally speaking, generic medications are less expensive than name-brand ones.

What are phentermine and topiramate?

Is phentermine a medication for weight loss?

One prescribed drug for weight loss is phentermine. In 1959, the Food and Drug Administration (FDA) authorized its usage for those over 16 for a maximum of 12 weeks. Phentermine was mixed with other weight-loss medications by pharmaceutical companies in the 1990s. Fen-phen was the common name for this combination of drugs.

Reference

Phentermine: Drug Uses, Dosage & Side Effects. (n.d.). Drugs.com. https://www.drugs.com/phentermine.html
Wikipedia contributors. (2025a, February 17). Phentermine. Wikipedia. https://en.wikipedia.org/wiki/Phentermine
Phentermine (Adipex-P, Lomaira): Uses, side effects, interactions, pictures, warnings & dosing – WebMD. (n.d.). https://www.webmd.com/drugs/2/drug-4151/phentermine-oral/details
Cpt, K. D. M. R. (2023, February 3). Does phentermine work for weight loss? A diet pill reviewed. Healthline. https://www.healthline.com/nutrition/phentermine-weight-loss#bottom-line