Phentermine vs. tirzepatide: which is better for weight loss?

Comparing phentermine vs. tirzepatide makes sense since they’re both used for weight loss. But the two drugs work in different ways and aren't interchangeable. Below, we compare what the clinical trial data and their approved labels actually show, covering results, side effects, cost, safety, and how long you can take them.

In a large trial of 2,539 adults, people taking tirzepatide lost an average of 15%--20.9% of their body weight over 72 weeks. Those taking the highest dose of tirzepatide, 15 mg per week, saw the greatest weight loss results. 

In a separate trial, people taking phentermine at its highest dose, 37.5 mg a day, lost an average of 9.3% over 14 weeks.

Phentermine was studied in a much smaller trial and over a shorter period, because it's only approved for short-term use. The drug’s prescribing information (i.e. the label approved by the US Food and Drug Administration, or FDA) notes that tolerance to the appetite-suppressing effect usually develops within a few weeks.

All that’s to say, these figures aren't from a direct head-to-head comparison of tirzepatide vs. phentermine. What they do show is that tirzepatide keeps working over a far longer period.  

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Phentermine and tirzepatide work through different pathways in the body.

Phentermine stimulates the central nervous system (CNS), triggering the release of norepinephrine, a chemical messenger in the brain that can help reduce appetite. It works in a similar way to amphetamines (think: Adderall) and carries some potential for misuse. Though it is a controlled substance, the risk of dependence in clinical practice is lower when compared to other medications such as opioids. 

Tirzepatide, which is available as brand-name Zepbound for obesity and Mounjaro for type 2 diabetes, works on your hormones instead.

It activates two hormonal pathways, glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), which together slow digestion, reduce appetite, and improve how your body handles blood sugar. GLP-1 regulates appetite and how much you eat, and nonclinical studies suggest GIP may add to that effect. It's taken as a once-weekly injection, not a daily pill.

If a daily pill is what you're after, there are now FDA-approved oral GLP-1 options for weight management in eligible adults: the Wegovy pill (oral semaglutide) and Foundayo (orforglipron).

The practical difference is where each drug acts: phentermine on the brain's hunger signals, tirzepatide on the gut hormones that control appetite and digestion.

*Terms and conditions may apply for certain doses; see more here 

Tirzepatide and phentermine have different side effect profiles, and for some people, those differences can be the deciding factor.

Tirzepatide side effects that are most frequently reported include:

• Nausea, vomiting, and diarrhea (especially early in treatment)

• Constipation

• Abdominal pain or indigestion

• Injection site reactions

• Fatigue

Tirzepatide also carries an FDA boxed warning about the risk of thyroid C-cell tumors based on animal studies. It shouldn't be used by anyone with a personal or family history of medullary thyroid carcinoma (MTC) or multiple endocrine neoplasia syndrome type 2 (MEN 2), and it's not an option during pregnancy.

Nausea, vomiting, and diarrhea mostly happen early on, while you're working up to your maintenance dose, and ease off over time. 

Tirzepatide can also affect the heart. Its label reports a mean increase in heart rate of 1 to 3 beats per minute, alongside greater reductions in systolic blood pressure than placebo.

Phentermine's reported side effects include:

• Increased heart rate and blood pressure

• Dry mouth

• Insomnia and restlessness

• Headache and dizziness

• Constipation or diarrhea

The cardiovascular side effects are where phentermine requires the most caution. It can cause cardiovascular side effects. And its label notes the rare but very real possibilities of developing pulmonary hypertension or serious heart valve disease in some people who have taken the medication.

Anyone with a history of cardiovascular disease, uncontrolled hypertension (high blood pressure), hyperthyroidism, or glaucoma should not take it for the most part.

Taking tirzepatide and phentermine together is not usually recommended, and no clinical trials have looked at whether this combination is safe or effective. That makes any combined use uncharted territory and even potentially dangerous.

Phentermine’s prescribing information specifically advises against combining it with other weight loss drugs, and adding tirzepatide introduces potential risks that haven't been studied.

The right choice depends on your health history, how long you need treatment, and what your healthcare provider recommends.

Phentermine may be an option if you:

• Want a short-term aid to weight loss alongside diet and exercise changes

• Have a BMI of 30 or above (or 27 with a weight-related health condition)

• Don't have cardiovascular disease, uncontrolled high blood pressure, or a history of substance misuse

• Are looking for an oral short-term treatment 

Phentermine is also available combined with topiramate as Qsymia, which is approved for long-term use. In a 28-week trial that compared the combination directly against each drug on its own, the combination produced greater weight loss than phentermine alone at the same dose, and the higher dose of the combination reduced body weight by 9.2% compared with 1.7% on placebo.

Tirzepatide may be an option if you:

• Need long-term weight management

• Have obesity or overweight and other conditions like type 2 diabetes, high blood pressure, or sleep apnea

• Haven't seen results with other approaches, including phentermine or other weight loss medications

• Don't have a personal or family history of medullary thyroid carcinoma (MTC) or multiple endocrine neoplasia syndrome type 2 (MEN 2)

Tirzepatide dosing starts at 2.5 mg once weekly, increasing to 5 mg after four weeks, up to a maximum of 15 mg once weekly. Phentermine is usually prescribed at 37.5 mg once daily (though your dose may vary), taken before breakfast or 1–2 hours after. Dosage should be discussed with your healthcare provider and not something you should handle on your own.

Neither medication works in isolation. Both are intended to be used alongside a reduced-calorie diet and regular physical activity, and any decision to start, adjust, or stop either one should involve your healthcare provider.

Yes, you can switch from phentermine to tirzepatide. Moving to a long-term option is a common question once phentermine's effect starts to wear off.

Phentermine's label says it should be stopped once tolerance to the appetite-suppressing effect develops. Tirzepatide works differently: It's approved to reduce weight and maintain that reduction long term, with no defined stopping point.

Phentermine's label also advises against taking it alongside other weight-loss medications, so switching generally means stopping phentermine first rather than taking both for a period of time. Your healthcare provider can advise on timing based on your dose and how long you've been taking it. They may also consider your other health conditions and whether you have been experiencing side effects. Don't stop or lower your dose on your own.

If you've hit a plateau on phentermine, talk to your healthcare provider about whether tirzepatide is the right next step for you.

Generic phentermine costs around $16–$20 for 30 tablets, making it one of the more affordable weight-loss medications available.

Zepbound’s price ranges from $299 to $1,086.37 per fill. Prices vary based on factors such as insurance coverage and the form prescribed. For example, the Zepbound KwikPen costs $299–$449 per month*, depending on the dose, when you pay cash on Ro. 

But if you opt to pay out of pocket elsewhere, you may have to pay over $1,000, given that single-dose pens of Zepbound have a list price of $1,086.37. 

*Terms and conditions may apply for certain doses; see more here.  

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When comparing phentermine vs. tirzepatide, know that while both medications can support weight loss, they just suit very different situations and have different safety profiles. One is built for a short push and eventually wears off; the other is designed to be taken for as long as it keeps working. Here’s a bit more about how the two stack up: 

• Tirzepatide produces greater weight loss than phentermine. In clinical trials, tirzepatide led to an average weight loss of up to 20.9% over 72 weeks, compared with 9.3% for phentermine over 14 weeks.

• Phentermine is designed to be taken for the short term. Appetite suppression tends to fade as your body adjusts, which is why it's only approved for short-term use.

• The side effects of phentermine and tirzepatide are different. Tirzepatide mostly causes nausea and other digestive issues, which can usually ease over time. Phentermine is more likely to affect heart rate and blood pressure.

• Phentermine carries a greater risk for heart health than tirzepatide. However, tirzepatide has a greater potential for gastrointestinal side effects and should not be taken in those with a personal or family history of medullary thyroid cancer. 

• Combining the two medications is not usually recommended and should only be done under the care and supervision of a healthcare provider (that is, if at all). There's no clinical trial data to support taking phentermine and tirzepatide together.

If you're weighing your options, Ro connects you with licensed healthcare providers who can review your full health history and help you figure out which approach makes the most sense for you.