Phentermine (Adipex-P) is an appetite suppressant. It's a prescription weight loss pill for people ages 17 years and older. This medication works best when it's part of a treatment plan that involves healthy diet and regular exercise. Phentermine (Adipex-P) is taken by mouth once daily, typically in the morning. Some of the more common side effects of this medication include dry mouth, trouble sleeping, and headache.
Short-term treatment for weight management in adults with a body mass index (BMI) of:
30 kg/m2 or greater; or
27 kg/m2 or greater, and with at least one other weight-related condition (e.g., high blood pressure, high blood sugar, high cholesterol)
Phentermine (Adipex-P) is a sympathomimetic (stimulant). The exact way it works to help with weight loss isn't well understood. But it's thought to work by lowering your appetite and causing you to feel less hungry.
Phentermine (Adipex-P) can cause some serious health issues. This risk may be even higher for certain groups. If this worries you, talk to your doctor or pharmacist about other options.
There isn't enough information to know whether it's safe to take phentermine (Adipex-P) along with other medications for weight loss, including ones found over-the-counter. Don't take phentermine (Adipex-P) with any other weight loss medication without first speaking with your primary care provider. Talk with your care team if you've more questions about your treatment plan.
Phentermine (Adipex-P) might not continue to work well after a while, even for people who've experienced good results when they first started treatment. This is called tolerance. Raising the dose of phentermine (Adipex-P) won't help and isn't safe because of the risk for serious side effects. Let you primary care provider know if you've noticed weight gain or your appetite returning after you've been taking phentermine (Adipex-P). They'll need to recommend other treatment options for you.
Risk factors: Taking more phentermine (Adipex-P) than prescribed
Similar to other stimulants, phentermine (Adipex-P) has a risk of misuse and for causing dependence. To lower the risk of serious side effects from taking too much medication and the risk of dependence, your primary care provider will typically only prescribe a limited amount and quantity at a time. Take phentermine (Adipex-P) exactly as prescribed by your primary care provider. This medication is only meant for short-term treatment only. Talk with your care team to learn more about long-term weight management options.
Risk factors: Drinking alcohol
Don't drive or do anything that needs a lot of focus and attention after starting phentermine (Adipex-P) until you know how this medication affects you. Some people experience trouble concentrating, dizziness, tremor and other side effects that can make it dangerous for you to drive. Let your care team know right away if you have these side effects, and especially if they don't get better with time. They might need to ask you to consider other treatment options instead.
In the past, there were rare, life-threatening cases of pulmonary hypertension (high blood pressure in the lungs) and heart valve problems in people taking "fen-phen" (fenfluramine / phentermine), a previously used combination weight management medication. Although it's unlikely for these problems to happen while taking phentermine (Adipex-P) by itself, these risks might still be possible. Tell your primary care provider right away if you have any symptoms of lung problems or heart valve problems, such as chest pain, shortness of breath, swollen feet, or dizziness.
Let your primary care provider (PCP) know if you have high blood pressure, even if your condition is well-controlled by medications. Taking phentermine (Adipex-P) might raise your risk for high blood pressure so your care team might ask you to check you blood pressure more often during treatment. Let your PCP know if you notice your blood pressure rising after starting treatment because they might need to adjust your medications.
Risk factors: Taking insulin or sulfonylureas
People with diabetes who are taking medications that lower blood sugar, such as insulin or sulfonylureas like glipizide, are at higher risk for severely low blood sugar during treatment with phentermine (Adipex-P). This can happen due to changes in eating habits and appetite with phentermine (Adipex-P). Your diabetes care team might need to adjust the dose of your blood sugar medications after you start phentermine (Adipex-P).
Always keep emergency glucose tablets or hard candy with you in case your blood sugar drops too low. If you experience symptoms of low blood sugar, such as feeling faint, shaking, and sweating, check and treat your low blood sugar levels right away. If your symptoms don't improve, call 911.
Source: DailyMed
Along with its needed effects, a medicine may cause some unwanted effects. Although not all of these side effects may occur, if they do occur they may need medical attention.
Check with your doctor immediately if any of the following side effects occur:
Rare
Seeing, hearing, or feeling things that are not there
severe mental changes
Incidence not known
decreased ability to exercise
dizziness
fainting
fast, irregular, pounding, or racing heartbeat or pulse
headache
numbness or tingling in the arms or legs
swelling of the feet or lower legs
trembling or shaking of the legs, arms, hands, or feet
trouble breathing
trouble with thinking, speaking, or walking
weakness
Some side effects may occur that usually do not need medical attention. These side effects may go away during treatment as your body adjusts to the medicine. Also, your health care professional may be able to tell you about ways to prevent or reduce some of these side effects. Check with your health care professional if any of the following side effects continue or are bothersome or if you have any questions about them:
Incidence not known
Decreased interest in sexual intercourse
difficulty having a bowel movement (stool)
false or unusual sense of well-being
hives or welts, itching, or skin rash
inability to have or keep an erection
increased in sexual ability, desire, drive, or performance
increased interest in sexual intercourse
loss in sexual ability, desire, drive, or performance
redness of the skin
unpleasant taste
Other side effects not listed may also occur in some patients. If you notice any other effects, check with your healthcare professional.
Call your doctor for medical advice about side effects. You may report side effects to the FDA at 1-800-FDA-1088.
Can help you lose 5% to 10% of body weight
Usually taken by mouth once per day
Available as a lower-cost generic
Comes as tablets, capsules, and orally disintegrating tablets
Most insurance companies don't cover the cost of phentermine (Adipex-P).
Has a risk of misuse and dependence since it's a stimulant medication that's similar to amphetamines
Might not be a good choice for people with high blood pressure
Should be avoided in children 16 years of age and younger, pregnant and breastfeeding women, and people with serious heart problems
Make sure you know which form (e.g., brand or generic, capsules or tablets) of phentermine you have. Each form can have different strengths of phentermine, dosing, and instructions on how to take them. Ask your pharmacist if you aren't sure which form you have or how to take the medication correctly.
Avoid taking phentermine (Adipex-P) close to bedtime because it can cause you to have trouble falling asleep.
Take phentermine (Adipex-P) exactly as prescribed. Don't change your dose or suddenly stop taking the medication without talking to your prescriber first.
Common side effects of phentermine (Adipex-P) are dry mouth and trouble sleeping. It can also raise your blood pressure and heart rate. Speak with your healthcare team if you experience any bothersome side effects while taking phentermine (Adipex-P).
You're recommended to avoid alcohol while taking phentermine (Adipex-P). Alcohol can worsen phentermine (Adipex-P) side effects like faster heart rate, higher blood pressure, and trouble sleeping.
Avoid driving the first few days after you start phentermine (Adipex-P) and after dose changes until you know how this medication affects you.
It's important that you eat a healthy diet and maintain a regular exercise routine while taking phentermine (Adipex-P). Doing so will help you get the most benefit out of the medication. Talk with your primary care provider if you have questions about how to plan your meals and what kind of exercise is appropriate for you.
Don't take phentermine (Adipex-P) if you're pregnant. Weight loss during pregnancy can cause serious harm to unborn babies. If you're pregnant or thinking of getting pregnant, don't use phentermine (Adipex-P). Speak with your primary care provider about how to maintain a healthy weight during pregnancy.
It's recommended to avoid breastfeeding while taking phentermine (Adipex-P) because there's a potential risk of harm to nursing babies. Phentermine (Adipex-P) is structurally similar to amphetamines, which are known to pass into breast milk and can have potentially serious risks (e.g., changes in blood pressure and heart rate, growth problems) to breastfed babies.
Phentermine (Adipex-P) interacts with several medications, including monoamine oxidase inhibitors (MAOIs). Tell your primary care provider about all the medications you're taking before you start phentermine (Adipex-P) as well as medications you plan to take during treatment. They can make sure your medications are safe for you.
For the phentermine (Adipex-P) orally disintegrating tablets (ODTs)
Leave the tablets in the blister pack until you're ready to use them. When you're ready, use dry hands to remove them. This helps prevent the tablets from dissolving from any moisture on your hands. Also don't push the tablets through the foil because the tablets can break.
Place the tablet on your tongue, and let the tablet dissolve without crushing or chewing it.
Phentermine (Adipex-P) 37.5 mg tablets: The typical dose is 18.75 mg to 37.5 mg by mouth once per day, either before breakfast or 1 to 2 hours after breakfast. Another option is 18.75 mg by mouth twice per day.
Phentermine 37.5 mg capsules: The typical dose is 37.5 mg by mouth once per day, either before breakfast or 1 to 2 hours after breakfast.
Phentermine 15 mg and 30 mg capsules: The typical dose is 15 mg to 30 mg by mouth once per day 2 hours after breakfast.
Phentermine 15 mg, 30 mg, and 37.5 mg orally disintegrating tablets (ODTs): The typical dose is 1 tablet dissolved on top of the tongue and swallowed once per day in the morning, with or without food.
Your provider will adjust your dose based on how well the medication is working for you, if you're having side effects, and if you have kidney problems.
GoodRx has partnered with PatientsLikeMe to provide reviews on the different aspects of Phentermine (Adipex-P).
Learn more about the effectiveness of Phentermine (Adipex-P) based on real life experiences.
Based on 13 people taking this medication
3
2.0
3.1
1.3
3.8
5.0
Severity of side effects
Based on 78 people taking this medication
0%
50%
100%
Insomnia
9%
Dry mouth
7%
Jittery feeling
6%
Anxious mood
4%
Constipation
4%
Reasons people stopped taking Phentermine (Adipex-P)
Based on 77 people who have taken this medication
Doctor's advice
21%
Other
21%
Did not seem to work
20%
Side effects too severe
15%
Course of treatment ended
13%
Expense
11%
Change in health plan coverage
2%
Using this medicine with any of the following medicines is not recommended. Your doctor may decide not to treat you with this medication or change some of the other medicines you take.
Using this medicine with any of the following medicines is usually not recommended, but may be required in some cases. If both medicines are prescribed together, your doctor may change the dose or how often you use one or both of the medicines.
Heart problems (e.g., stroke, irregular heartbeat, uncontrolled high blood pressure)
Overactive thyroid (hyperthyroidism)
Feeling agitated or distressed
History of substance use disorder
Currently pregnant or breastfeeding
Currently take or have taken a monoamine oxidase inhibitors (MAOI) in the last 14 days
Severe allergy to amphetamine-containing products
Short-term treatment for weight management in adults with a body mass index (BMI) of:
30 kg/m2 or greater; or
27 kg/m2 or greater, and with at least one other weight-related condition (e.g., high blood pressure, high blood sugar, high cholesterol)
30 kg/m^2^ or greater or
27 kg/m^2^ or greater and at least one weight-related condition (e.g., high blood pressure, type 2 diabetes, or high cholesterol)
Weight loss and long-term weight management in:
Lowering the risk of heart attack, stroke, and death due to heart problems in adults with a larger body size and heart disease
By providing your email, you consent to receive marketing communications from GoodRx, which may include content and/or data related to men’s health, women's health, reproductive care, or sexual health. You agree to the GoodRx Terms of Use and acknowledge the Privacy Policy. You can unsubscribe at any time.
American Lung Association. (2023). Pulmonary arterial hypertension symptoms and diagnosis.
American Thyroid Association. (n.d.). Hyperthyroidism.
Apovian, C. M., et al. (2015). Pharmacological management of obesity: An Endocrine Society clinical practice guideline. The Journal of Clinical Endocrinology & Metabolism.
Centers for Disease Control and Prevention. (1997). Cardiac valvulopathy associated with exposure to fenfluramine or dexfenfluramine: U.S. Department of Health and Human Services interim public health recommendations, November 1997. Morbidity and Mortality Weekly Report.
Centers for Disease Control and Prevention. (2022). Body mass index (BMI).
Coulter, A. A., et al. (2019). Centrally acting drugs for obesity: Past, present, and future. Drugs.
Drugs and Lactation Database (LactMed). (2022). Phentermine. National Institute of Child Health and Human Development.
Drugs and Lactation Database (LactMed). (2023). Amphetamine. National Institute of Child Health and Human Development.
Hendricks, E.J., et al. (2012). Blood pressure and heart rate effects, weight off and maintenance during long-term phentermine pharmacotherapy for obesity. Obesity.
KVK-Tech, Inc. (2018). LOMAIRA- phentermine hydrochloride tablet [package insert]. DailyMed.
Lannett Company, Inc. (2017). PHENTERMINE HYDROCHLORIDE capsule [package insert]. DailyMed.
Sun Pharmaceutical Industries, Inc. (2020). PHENTERMINE HYDROCHLORIDE tablet [package insert]. DailyMed.
Sunrise Pharmaceutical, Inc. (2022). PHENTERMINE HYDROCHLORIDE capsule [package insert]. DailyMed.
Teva Pharmaceuticals USA, Inc. (2020). ADIPEX-P- phentermine hydrochloride tablet. ADIPEX-P- phentermine hydrochloride capsule [package insert]. DailyMed.
U.S. Drug Enforcement Administration. (n.d.). Amphetamines.
U.S. Food and Drug Administration. (2021). Some medicines and driving don’t mix.
Wadden, T. A., et al. (1998). The fen-phen finale: A study of weight loss and valvular heart disease. Obesity Research.
Zydus Pharmaceuticals USA Inc. (2022). PHENTERMINE HYDROCHLORIDE tablet, orally disintegrating [package insert]. DailyMed
Research prescriptions and over-the-counter medications from A to Z, compare drug prices, and start saving.