Many people are familiar with the weight loss medication phentermine. Phentermine has been used in weight loss therapy since the mid-1900s and was approved for long-term use in 2012.

Semaglutide is new to the neighborhood as a weight loss drug, receiving its FDA approval in 2021. When choosing which medication is right for you, the tried and true or the new kid on the block, you may wonder – semaglutide vs. phentermine – which is the better medication?

What kind of medication is phentermine?

Phentermine is a central nervous stimulant and “anorectic” (hunger suppressing) medication. Taking phentermine increases the neurotransmitters in the brain that give us energy and decrease hunger.

What kind of medication is semaglutide?

Semaglutide belongs to a class of medication called GLP-1. This medication mimics a peptide our body naturally makes called glucagon-like peptide-1 (GLP-1). GLP-1 is secreted by the intestines and has multiple functions within the body. Semaglutide is 94% like the peptide already made naturally by our body.

How does phentermine cause weight loss?

Phentermine causes weight loss primarily through the loss of appetite.

In our clinics, some patients do report a positive increase in energy and mood which they find helpful with motivation to make healthier choices such as cooking at home versus eating out or going for regular walks instead of remaining sedentary.  We even have the occasional patient that feels phentermine helps to alleviate their personal symptoms of depression which helps in their weight loss efforts.  Of course, these additional effects listed above may or may not come to be your personal experience.¹

How does semaglutide cause weight loss?

The exact way that semaglutide causes weight loss is unknown. However, three important functions are altered in people taking semaglutide that may be the primary contributing factors to weight loss.

1. GLP-1 medications slow down the movement of the intestinal tract. Keeping food in the stomach longer keeps you full for longer.
2. Act on the brain to decrease hunger, increase satisfaction, and alter the food-reward system (changing food preferences in favor of healthier options.)
3. Improves the body’s response to insulin, a term called “insulin sensitivity,” which positively affects blood sugar levels and weight loss.²

What kind of side effects does phentermine cause?

The most common side effects are dry mouth, jitters, increased heart rate, anxiety, irritability, constipation, and insomnia. Most patients only experience a handful of side effects, if any, and these are usually mild and well-tolerated.

Dry mouth, constipation, and occasionally insomnia may persist throughout treatment. Jitters, anxiety, and irritation usually either decrease in the severity or go away completely at some point during the treatment.³

More serious adverse effects can be life-threatening, and symptoms should not be ignored. Harmful effects on the cardiovascular system have been reported.

They include – increased blood pressure, tachycardia (heart rate over 100 beats per minute), pulmonary hypertension, disease of the heart valves, or even events related to ischemia (loss of blood flow) to the heart or other organs. Ischemia of the heart can lead to heart attacks, and ischemia in the brain causes a stroke.¹

What kind of side effects does semaglutide cause?

The most common side effects we see in clinic are mild nausea, diarrhea or constipation, burping, and headache. Again, many patients only experience one or two side effects and these are usually mild and well-tolerated. We rarely have patients quit semaglutide due to side effects.

It is rare for people to vomit, but it happens if the patient is sensitive to the medication, or the dosage is increased too quickly.⁴

More serious side effects were reported in trials and include acute pancreatitis, acute gallbladder disease (which may occur without the use of any medication if the rate of weight loss is very rapid), hypoglycemia, acute kidney injury (because of dehydration) and increased

Who shouldn’t take phentermine?

Most people tolerate phentermine well and can take the medication as long as any other health conditions are well managed under the care of a medical provider. If you have any of the following conditions, it is best to proceed only after a thorough medical history is discussed with your phentermine prescriber.

People with a history of cardiovascular disease are at much higher risk of severe adverse effects than those without. Cardiovascular disease is an umbrella term that includes multiple conditions, including coronary artery disease, stroke, arrhythmias, congestive heart failure, and uncontrolled high blood pressure.

Patients who take monoamine oxidase inhibitors.

People with a history of untreated hyperthyroidism or glaucoma.

People who are pregnant or planning to get pregnant.¹

Who shouldn’t take semaglutide?

In animal trials, mice and rats given semaglutide had an increased incidence of thyroid C-cell tumors.

Humans have not reported this trend, so it is unclear whether to consider this a risk. However, it is advised that anyone with a history of medullary thyroid cancer proceed with caution and have regular thyroid monitoring if they choose to take semaglutide.

Finally, pancreatitis was reported during the clinical trials. Therefore, anyone with a history of pancreatitis should be familiar with the signs and have a thorough medical exam before starting semaglutide.⁵

People who are pregnant or planning to get pregnant.

How is phentermine administered?

Phentermine has two primary formulations. The first is a 37.5 mg tablet, and the second is a 30 mg extended-release capsule.  Most people take 1 tablet or capsule per day.

Your provider will determine the appropriate dosage based on your medical history and weight loss goals.¹

How is semaglutide administered?

Semaglutide is a once-weekly injection that is either administered via an autoinjector pen or drawn up into a syringe and injected. The medication is injected into a fatty portion of the arm, stomach, or thigh.

The dosing schedule begins with a small introductory dosage of 0.25 mg and is increased slowly until a therapeutic dosage (a dose which provides good hunger suppression, minimal side effects, and consistent weight loss) is reached or the highest FDA-approved dose, 2.4 mg, is reached.  At Rivas we are able to provide custom doses for our patients to further individualize the effectiveness of treatment.⁵

How much weight can I lose with phentermine?

It is important to note that most research performed on phentermine was conducted when the approved length of treatment was 3 months. Phentermine has now been approved for chronic weight management.

Many people in our practice have lost their desired amount using phentermine.  This amount ranges from 20 – 150 pounds or more.  Results are varied and do depend on lifestyle modifications that promote a healthful lifestyle as well as individual responsiveness to the medication.

How much weight can I lose with semaglutide?

Four trials were completed on the effectiveness of semaglutide. After the first 3 studies, results were reported as follows:

1. The average weight loss percentage was -13.5% of starting weight among 2,117 participants. For example, in the average person weighing 200 lbs. at starting, this would be a 26 lb. weight loss.
2. 1% of participants lost more than 15% of their starting weight. For example, in a person weighing 200 lbs. at starting, this would be a 30 lb. weight loss or more.⁵

In practice, we routinely see patients lose 20% or more of their starting weight using semaglutide.   We began offering semaglutide in the fall of 2021 (1 year at the writing of this article) and many patients have lost their desired amount which ranges from 30 – 70 pounds or more.  I do expect these numbers will increase with time and length of treatment.

Our results outperform the expected percentage from the trials and are usually achieved by those who adopt lifestyle modifications which promote a healthful lifestyle as well as individual responsiveness to the medication.

Can phentermine and semaglutide be taken together?

Yes! We have many patients who opt for dual therapy at some point during their treatment.  This approach has been very helpful in patients who have significant weight to lose, have plateaued in their efforts, or patients in whom the additional energy is beneficial to their life situation.

Which is the better choice?

The best medication choice fits your health profile, life situation, and financial budget.  If you need help deciding which medication is right for you, our providers will discuss your individual needs and help you determine the right choice.