What each drug is
Phentermine is a stimulant pill. It is related to amphetamine and works on the brain to lower appetite and give a small energy boost. The FDA approved it in 1959 for short-term use, usually a few weeks at a time. It is sold as a generic and under brand names like Adipex-P.
Tirzepatide is a weekly injection made by Eli Lilly. It is sold as Zepbound for weight loss and Mounjaro for type 2 diabetes. It copies two gut hormones, GLP-1 and GIP, which lower appetite and slow the stomach. It is approved for long-term use.
How much weight each one causes you to lose
Phentermine on its own typically leads to about 5 to 7 percent of body weight lost over three to six months. For a 220-pound person, that is roughly 11 to 15 pounds. Results drop off when the pill is stopped, and most studies were short.
Tirzepatide is in a different league. In the SURMOUNT-1 trial, people on the highest dose lost about 21 percent of their body weight over 72 weeks, roughly 46 pounds for a 220-pound person. More than half of people on the top dose lost 20 percent or more.
There has been no large head-to-head trial, but the gap between the two is wide enough that researchers consider tirzepatide the most effective weight-loss drug available today.
Side effects and safety
Phentermine is a stimulant, so its side effects are stimulant side effects: faster heartbeat, higher blood pressure, trouble sleeping, dry mouth, nervousness, and jitteriness. It should not be used by people with heart disease, uncontrolled high blood pressure, overactive thyroid, glaucoma, or a history of drug abuse. It is a controlled substance.
Tirzepatide mostly causes stomach side effects: nausea, diarrhea, vomiting, constipation. These are usually worst during the dose ramp and fade. Rare but serious risks include pancreatitis and gallbladder problems. It carries a boxed warning about thyroid C-cell tumors seen in rodents, so people with a family history of medullary thyroid cancer should not use it.
For most people, tirzepatide's side effects are easier to live with over the long term because they are not tied to the heart and nervous system.
How long you can take each one
Phentermine is approved for short-term use. Many clinicians prescribe it for a few months, stop, and restart. Longer use happens in practice but is off-label. Because it is a stimulant, there are concerns about tolerance, where the effect fades over time.
Tirzepatide is meant for ongoing use. Obesity is treated as a long-term condition, and the SURMOUNT-4 trial showed that people who stopped tirzepatide regained about half of their lost weight within a year, while those who stayed on it kept losing.
Cost and convenience
Generic phentermine is cheap, often under $30 a month. It is a once-a-day pill. That is its biggest advantage.
Brand Zepbound can cost over $1,000 a month without insurance. Compounded tirzepatide, which is made by a licensed pharmacy and is not FDA-approved, costs far less. At Zen IV Spa it is $299 per month. It is a once-weekly shot with a tiny needle that most people learn to do in a minute.
Which one is right for you?
Phentermine can make sense for someone who needs a modest, short-term push, has a healthy heart and blood pressure, and wants the lowest cost option. It is not a good fit for anyone with heart or anxiety issues.
Tirzepatide makes sense for someone who has a lot of weight to lose, wants a long-term plan, and can handle a weekly injection and some early stomach side effects. It is the stronger tool by a wide margin.
At Zen IV Spa, our medical director reviews your history, your blood pressure, and your goals in a free video visit. A nurse draws labs at your home in New York and New Jersey. We prescribe compounded tirzepatide for $299 per month, compounded semaglutide for $199 per month, and retatrutide for $249 per month. All are compounded and not FDA-approved. Learn more at /weight-loss/.
Common Questions
Which causes more weight loss, phentermine or tirzepatide?+
Tirzepatide, by a lot. Phentermine usually produces 5 to 7 percent weight loss. Tirzepatide produced about 21 percent at the top dose in its main trial.
Can I take phentermine and tirzepatide together?+
This is sometimes done off-label, but it is not well studied. Only a prescriber who knows your full history should decide.
Is phentermine safe for long-term use?+
It is approved for short-term use. Longer use is off-label and needs regular blood pressure and heart monitoring.
Why is phentermine so much cheaper?+
It is an old generic pill. Tirzepatide is a newer, more complex drug. Compounded tirzepatide narrows the gap but still costs more.
Does tirzepatide raise blood pressure like phentermine?+
No. Tirzepatide tends to lower blood pressure as weight comes off. Phentermine can raise it because it is a stimulant.
Is compounded tirzepatide the same as Zepbound?+
It contains the same active ingredient but is made by a compounding pharmacy and is not an FDA-approved product.
Sources: FDA prescribing information for Zepbound (tirzepatide), 2024, and for Adipex-P (phentermine); SURMOUNT-1 trial, Jastreboff et al., NEJM 2022; SURMOUNT-4 trial, JAMA 2024; Mayo Clinic overview of prescription weight-loss drugs.
All treatments are medical services. Every guest is screened, and every treatment is prescribed by a licensed provider. Nothing on this page is medical advice for you personally, and the opinions summarized here belong to the doctors named, not to Zen IV Spa. Talk to a provider before starting or stopping anything.
