Phentermine vs Semaglutide: A Comparison of Two Weight Loss Drugs

Weight loss drugs are an innovative solution to help an obese individual manage their weight and reduce the risk of resulting morbidity. With the spectrum of weight loss drugs marketed, it is difficult to choose between them. This article will closely examine Phentermine and Semaglutide, two drugs used for weight loss. We will explore their mechanisms of action, similarities, differences, and potential side effects. This scientific comparison will help you better understand these medications and their role in weight management.
Klara Hatinova

Klara Hatinova

Klara is postgraduate researcher in experimental psychology at the
University of Oxford.

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What are phentermine and semaglutide?

Phentermine and semaglutide are weight loss drugs, but they function in different ways and are meant to be used for different periods of time.

Phentermine is a stimulant, similar to Adderall, meaning that it stimulates your central nervous system towards a fight-or-flight response. Phentermine was approved by the FDA in 1959 and has since been sold under various brand names, including Adipex-P, Lomaira, and Suprenza [1]. Since 2012, it has been combined with topiramate, an anti-seizure drug that increases inhibitory neurotransmission in the brain [2].

The exact mechanism by which phentermine reduces appetite is unclear, but it increases norepinephrine levels in your brain, suppressing appetite and increasing metabolism.

This effectiveness of phentermine fades over time, so it is only approved for use for up to 12 weeks. Phentermine is, therefore, a controlled substance that can be addictive with a risk of withdrawal symptoms and cardiac side effects such as a racing or irregular heartbeat [3].

Semaglutide is a glucagon-like peptide-1 (GLP-1) receptor agonist, a peptide for weight loss. The active drug semaglutide is sold under the brand names Wegovy and Ozempic in injectable form and Rybelsus oral tablets. Semaglutide mimics the GLP-1 peptide, increasing insulin and thereby reducing blood sugar. Increases in insulin also signal sufficient fuelling to the brain, reducing appetite [4]. Semaglutide is designed for long-term use since people may experience weight gain if they stop using it.

Semaglutide does have side effects. For a more detailed analysis read our blog about the side effects of tirzepatide vs semaglutide

Phentermine vs Semaglutide: Similarities

Phentermine and semaglutide are both medications that can aid in weight loss. However, both medications are only effective when combined with lifestyle changes such as diet, exercise, and weight reduction behavioural modifications [3].

Both medications work by affecting the body's response to hunger. Semaglutide mimics the GLP-1 hormone, signalling fullness to your brain, while phentermine stimulates your central nervous system, reducing appetite [2].

Another similarity is that they are both available under various brand names. Semaglutide is sold under the brand names Wegovy, Ozempic, and Rybelsus, while phentermine is available as Adipex-P, Lomaira, and Suprenza [3].

Phentermine vs Semaglutide: What Are The Differences?

Outside of the common indication for weight loss, phentermine and semaglutide are very different medications with regards to how they work, how long they should be used for and the side effects they produce.

Semaglutide is a GLP-1 receptor agonist. It works by imitating the GLP-1 hormone, increasing insulin levels, lowering blood sugar, and signalling satiety to the brain. Semaglutide formulations are intended for long-term use and are safe for several years.

Possible side effects of semaglutide include nausea and vomiting, constipation or diarrhoea, headache, abdominal pain, fatigue, and hypoglycemia [3].

On the other hand, phentermine is a stimulant drug that stimulates norepinephrine and dopamine release in your central nervous system [1]. By increasing the activity of the sympathetic nervous system, phentermine reduces the activity of the rest-and-digest system, reducing appetite.

Phentermine should only be used for short periods because it can be addictive. This is because phentermine increases dopamine levels, increasing the perceived reward of taking the drug [1]. Therefore, the combination of phentermine and topiramate for weight loss should only be used for 12 weeks.

As a stimulant drug, the side effects of phentermine are distinct from semaglutide. Side effects of phentermine include insomnia, headache, accelerated or irregular heart rate, dry mouth, mood changes, hypertension, heart failure, hallucinations, and reduced sex drive [3]. These side effects are comparable to recreational drugs, such as tesla pills.

Studies on weight loss effectiveness have found that injectable semaglutide has a more significant effect on weight loss than phentermine [3]. However, the choice between these two medications should be made in consultation with a healthcare professional, considering the individual's specific health situation and the impact of the side effects.

Summary: Phentermine vs Semaglutide

Phentermine and semaglutide are very different weight-loss drugs and should be used for different periods of time. The duration of taking the medication and potential side effects are the primary differences that should be considered when deciding which drug to choose. In clinical studies, semaglutide resulted in more profound weight loss than phentermine and lower side effects, suggesting semaglutide should be the first line option discussed with your medical provider.

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Klara Hatinova

Klara Hatinova

Klara is a postgraduate researcher in experimental psychology at the University of Oxford. She has worked across a spectrum of hot topics in neuroscience, including her current project measuring reinforcement learning strategies in Parkinson’s disease. Previously, she studied the efficacy of psilocybin as a therapy for critical mental health conditions and examined molecular circadian rhythms of migraine disorders. She completed her undergraduate degree in Neuroscience at the University of Glasgow and participated in a year abroad at the University of California, where she worked on a clinical trial for spinal cord injury.