Key takeaways:
Phentermine (Adipex-P, Lomaira) is an effective oral weight-loss medication. It’s FDA approved for those with a body mass index (BMI) over 30. It’s also approved for those with a BMI over 27 who also have a weight-related health condition, such as high blood pressure or diabetes.
“Fen-phen” — a combination of phentermine and fenfluramine — caused dangerous side effects for some people. But phentermine on its own doesn’t carry the same risks and is considered safe.
Phentermine is only approved for short-term use (12 weeks or less). But your prescriber may be OK with you taking it longer based on your medical history.
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Newer weight-loss medications, such as semaglutide (Wegovy) and tirzepatide (Zepbound), are getting a lot of attention. But there are other options that have been around a lot longer and don’t require an injection.
Phentermine (Adipex-P, Lomaira) is an oral weight-loss medication that was FDA approved in 1959. But how does phentermine work for weight loss? And is it a safe and effective option? Let’s explore the answers to these and more common questions about phentermine to help you make informed choices about your weight-loss treatment options.
1. How does phentermine work for weight loss?
Phentermine is a stimulant medication. Stimulants help speed up the functions of your body.
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Phentermine works by activating your sympathetic nervous system, also known as your fight-or-flight response. This helps reduce your appetite, so you eat fewer calories. Over time, eating less can create an energy deficit — meaning you’re burning more calories than you’re eating — which can lead to weight loss. People also report a boost in energy when they’re taking phentermine, which helps them be more active.
Phentermine works best if it’s combined with regular exercise and a balanced diet. If you don’t make lifestyle changes while taking phentermine, it may not work as well for you. And you may have a higher chance of regaining weight once you stop taking it.
2. How much weight will you lose while taking phentermine?
How much weight you’ll lose while taking phentermine largely depends on two things: how much excess weight you have and how well you’re able to adjust your diet and exercise habits while taking it.
Experts consider losing 3% to 5% of your body weight to be a clinically meaningful change. This means it’s likely to have a positive impact on your overall health and well-being.
So how does phentermine measure up when looking at meaningful weightful weight loss? There are several studies that directly address this question:
A study of over 900 people from Mexico showed that about 80% of people living with obesity lost at least 5% of their body weight after 6 months of phentermine treatment.
One small study from South Korea showed that almost 86% of people taking phentermine for 14 weeks lost 5% or more of their body weight. And more than 50% of people lost 10% or more of their body weight. This study included people with a body mass index (BMI) of 25 or more.
Another South Korean study looked specifically at people considered obese (a BMI of 30 or higher) who took phentermine for 12 weeks. Almost 96% of study participants lost more than 5% of their body weight. And 62% of the same group lost more than 10% of their body weight.
Phentermine is also an ingredient in the prescription weight-loss medication Qsymia (phentermine / topiramate). A large study found that 75% of people taking Qsymia over a year lost at least 5% of their body weight, compared to 23% of people taking a placebo (a pill with no medication in it).
2. How much weight will you lose while taking phentermine?
How much weight you’ll lose while taking phentermine largely depends on two things: how much excess weight you have and how well you’re able to adjust your diet and exercise habits while taking it.
Losing 3% to 5% of your body weight is considered clinically meaningful. This means it’s likely to have a positive impact on your overall health and well-being.
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So how does phentermine measure up? There are several studies that directly address this question:
A study of over 900 people from Mexico showed that about 80% of people with obesity lost at least 5% of their body weight after 6 months of phentermine treatment.
One small study from South Korea showed that almost 86% of people taking phentermine for 14 weeks lost 5% or more of their body weight. And more than 50% of people lost 10% or more of their body weight. This study included people considered either overweight or obese.
Another South Korean study looked specifically at people considered obese (not overweight) who took phentermine for 12 weeks. Almost 96% of study participants lost more than 5% of their body weight. And 62% of the same group lost more than 10% of their body weight.
Phentermine is also an ingredient in the prescription medication Qsymia (phentermine / topiramate). A large study found that 75% of people taking Qsymia over a year lost at least 5% of their body weight, compared to 23% of people taking a placebo (a pill with no medication in it).
3. How long does it take for phentermine to work?
In the studies mentioned above, some people experienced weight loss within the first 2 weeks of taking phentermine. For others, it took more than 3 months to start noticing a change in body weight.
Your prescriber will likely check in with you to see how phentermine is working when you start taking it. If you aren’t noticing a change in your weight, they may increase your dosage. But you shouldn’t take more phentermine than prescribed. This could raise the risk of side effects.
4. Is phentermine safe?
Yes, phentermine is FDA approved for weight loss. This means it’s considered a safe and effective medication when you take it as prescribed. But there are some risks to be aware of.
Phentermine is a controlled substance. This means it carries a risk for misuse or dependence. Studies on phentermine show this isn’t likely to happen when phentermine is taken as prescribed for weight management. So as mentioned, it’s important to stick to the phentermine dosage your prescriber suggests.
The most common phentermine side effects are dry mouth, trouble sleeping, and headache. In some cases, it may cause tremors, changes in your mood or behavior, and an increased heart rate. Most phentermine side effects will go away as your body adjusts to the medication.
Who shouldn’t take phentermine?
Phentermine isn’t approved for people under age 17 or those with a BMI below 27. It’s also not recommended if you’re living with certain health conditions, including:
Heart disease
High blood pressure that isn’t well managed
Hyperthyroidism (too much thyroid hormone)
Glaucoma (high eye pressure)
High levels of nervousness or anxiety
History of drug misuse
Pregnancy
Breastfeeding
Phentermine may not be the best weight-loss medication for you if you consume alcohol regularly. The combination could increase your risk of alcohol-related harm and interfere with your weight-loss goals.
And phentermine isn’t recommended if you’re taking monoamine oxidase inhibitors (MAOIs). The combination could cause dangerously high blood pressure.
Good to know: Phentermine is only approved for people with a BMI of 30 or more (or 27 if you’re also living with a weight-related health condition such as diabetes or high blood pressure). But BMI isn’t a perfect tool and may not be an accurate measure of your health. Your healthcare team may decide to prescribe phentermine even if you don’t fall into one of these approved categories. But you shouldn’t take phentermine on your own without the guidance of a healthcare professional.
5. Does phentermine have the same side effects as ‘fen-phen’?
No, phentermine was an active ingredient in the medication known as “fen-phen.” This was a combination of two medications: fenfluramine and phentermine. Fenfluramine is another stimulant medication. But fenfluramine and fen-phen were never FDA approved for weight loss — they were prescribed off-label.
Unfortunately, the combination of these two medications can cause dangerous side effects. This includes damage to heart valves and a fatal lung condition called primary pulmonary hypertension (PPH). Due to these side effects, the FDA recommended removal of fenfluramine from the market in 1997.
There have been no reported cases of heart valve disease related to phentermine alone. But there have been rare cases of PPH reported with phentermine alone. Still, this side effect is extremely rare and the risk is much lower with phentermine than with fen-phen.
Good to know: Fenfluramine was re-approved by the FDA in 2020 for a certain type of seizure disorder. It now has a required safety program to make sure it’s only prescribed for approved conditions and to lower the risk of heart and lung problems from taking it.
6. What’s the best time to take phentermine?
Since phentermine can have stimulating effects, it’s best to take it in the morning, an hour or two after eating breakfast.
If you take phentermine tablets, your prescriber may have you divide your dose into two smaller daily doses. This can help with afternoon appetite control if needed. And if you take Lomaira (a low-dose phentermine tablet) you’ll take it 3 times a day, 30 minutes before meals.
Regardless of which form of phentermine you take, you should avoid taking it in the evening. It could interfere with your sleep if you take it too late in the day.
7. How long does phentermine stay in your system?
Phentermine’s half-life is about 20 hours. This means it takes about a day to remove half of your phentermine dose from your body after taking it. It takes about 4 days for your body to fully clear phentermine after your last dose.
Does phentermine cause withdrawal symptoms when stopped?
No, phentermine doesn’t seem to cause withdrawal symptoms when you stop taking it. This is a common concern, since withdrawal symptoms can happen with other stimulant medications. But it doesn’t seem to be an issue with phentermine.
8. How long can you take phentermine?
Phentermine is only approved for short-term use (12 weeks or less). This is because there wasn’t any data that showed it was safe to take long term when it was initially approved. But newer studies show that long-term phentermine use (more than 3 months) is not only safe, but also helps people lose more weight.
Your healthcare team will help you decide how long to take phentermine based on your medical history and how well it’s working for you.
How to save on the cost of phentermine
Phentermine is one of the most commonly prescribed oral weight-loss medications. This is largely due to the fact that it is also one of the most affordable options. GoodRx can help you navigate ways to save on the cost of your phentermine prescription:
Phentermine capsules are only available as a generic medication. GoodRx can help you pay as little as $14.54 for a 30-day supply.
Adipex-P is available as both a brand-name and lower-cost generic. You can pay as little as $10.57 for a 30-day supply of the generic with GoodRx.
Lomaira is only available as a brand-name medication. You can pay as low as $17.63 per month with a free GoodRx discount. If you have commercial insurance, you can pay as little as $0.50 per pill with a copay card from the manufacturer.
The bottom line
Phentermine (Adipex-P, Lomaira) is an effective oral weight-loss medication. Phentermine works for weight loss by reducing your appetite. You may also feel a boost of energy while you’re taking it. In studies, at least 80% of participants lost at least 5% of their body weight while taking phentermine. But phentermine works best if you combine it with regular exercise and a reduced-calorie diet.
Phentermine is only FDA approved for short-term use (12 weeks or less). But studies show it’s likely safe to take it long-term if your prescriber OKs it.
Phentermine is considered safe, but it has risks to be aware of. If you’re living with heart disease, have a history of substance misuse, or consume alcohol regularly, it may not be the best weight-loss medication for you. Common phentermine side effects include dry mouth and trouble sleeping. It’s best to take phentermine in the morning unless your prescriber tells you otherwise. This will help prevent it from causing sleep issues.
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References
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