Key takeaways:
Phentermine (Adipex-P, Lomaira) is an oral weight-loss medication. Common phentermine side effects include dry mouth, trouble sleeping, and headache. Tremor, stool changes, and sexual side effects are also possible.
Rare, but serious phentermine side effects include mood and behavior changes and increased blood pressure and heart rate. Phentermine is not recommended for people who have heart problems or a history of substance misuse.
Life-threatening side effects, such as heart valve disease and high blood pressure in the lungs (pulmonary hypertension), are extremely rare when taking phentermine on its own.
It can be challenging to maintain a healthy body weight with lifestyle changes alone. Phentermine (Adipex-P, Lomaira) can be a short-term solution for people who need a little extra help losing weight. It's an oral tablet that works best in combination with good nutrition, exercise, and behavioral changes.
If your healthcare team recommends phentermine, you may be curious about how you’ll feel while you’re taking it, including what side effects it may cause.
Phentermine side effects at a glance
Some phentermine side effects will go away as your body adjusts to the medication. Others are more serious, so they’re good to be aware of so you know when to seek medical care. Here are some of the common and serious phentermine side effects reported in clinical trials. Keep in mind that everyone responds to medication differently, so you may experience a side effect that isn’t listed here.
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Common phentermine side effects include:
Dry mouth
Trouble sleeping
Headache
Tremor
Dizziness
Restlessness
Constipation or diarrhea
Small increase in heart rate and blood pressure
Euphoria (feeling intoxicated or “high”)
Sexual side effects
Taste changes
Itchy skin or hives
Rare but serious phentermine side effects include:
Dangerously high blood pressure or heart rate
Pulmonary hypertension (a rare lung condition)
Damage to the heart valves
Misuse and dependence
Agitation, hallucinations, or other mood and behavior changes
Let’s take a closer look at nine possible phentermine side effects and what to do if you experience them.
1. Dry mouth
Dry mouth is one of the most common phentermine side effects. That’s because it can decrease saliva production. With less saliva, your mouth can feel drier than normal. Not only is this unpleasant, but chronic dry mouth can also lead to difficulty swallowing and tooth decay.
Make sure to drink plenty of fluids while taking phentermine. Try to avoid foods that are overly sweet or salty — they can worsen dry mouth. You can also try an over-the-counter (OTC) dry mouth treatment such as gum, mouthwash, or lozenges.
If this side effect worsens or doesn’t get better over time, let your prescriber know. They can help determine if phentermine is the cause and how best to manage it.
2. Insomnia
Insomnia (trouble sleeping) is another common side effect of this medication. Phentermine is considered a stimulant, and it can cause you to feel more alert and energetic. This can make it harder for you to fall and/or stay asleep.
You can help prevent sleep issues by taking phentermine once daily in the morning. Taking it early in the day gives your body more time to process the medication. By the time you’re ready to go to bed, there’s less medication in your system to affect your sleep.
It’s also helpful to practice good sleep hygiene. Avoid alcohol, caffeine, and heavy meals before bed. Create a relaxing environment, and try to go to bed at the same time every night.
Sleep issues may resolve once your body adjusts to phentermine. But if they continue, talk to your prescriber. They may lower your dose or add a different medication to help you sleep.
3. Headache
Some people taking phentermine experience headaches. Phentermine can constrict and relax your blood vessels. This can change the blood flow going to your brain, which can activate pain signals and lead to headaches.
Keeping yourself hydrated may help. You can also ask your prescriber or pharmacist about OTC treatment options, such as acetaminophen (Tylenol) or ibuprofen (Advil, Motrin).
If you notice that your headaches aren’t going away or becoming more severe, talk to your healthcare team. Persistent headaches could be a sign of a more serious health problem and should not be ignored.
4. Tremor
Since it’s a stimulant, phentermine can sometimes cause tremors, or involuntary shaking. Although this side effect is usually mild and goes away quickly, it could also be a sign that your dose is too high.
Let your prescriber know if you’re experiencing tremors that aren’t improving. They may need to lower your phentermine dose. But if you have other symptoms along with a tremor, like fast breathing, confusion, and stomach cramps, seek medical care right away. These could be signs that you’ve taken too much phentermine.
5. Behavioral changes
Phentermine releases certain chemicals in the brain that can lead to a euphoric or “high” feeling. Since phentermine can alter your state of mind, you should avoid driving or other tasks that require coordination until you know how it will affect you. You should also avoid drinking alcohol if you’re taking phentermine, as it can worsen these effects.
In rare cases, phentermine may cause more severe mood changes, such as feeling overly agitated or hallucinating (seeing things that aren’t there). If you start to experience changes in your mood or behavior while taking phentermine, be sure to let your healthcare team know.
Because of its effects on the brain, phentermine is categorized as a controlled substance. This means it has a potential for misuse and dependence. That being said, it seems that phentermine may actually have a low potential for dependence when used properly for weight loss. Your prescriber will be able to determine whether phentermine is a good choice for you based on your medical history.
6. Constipation or diarrhea
Phentermine may cause constipation. But some people also reported diarrhea while taking it in clinical studies.
Whether you experience constipation or diarrhea while taking phentermine, hydration is important. Fluids can help keep things moving through your digestive tract if you have constipation. They can also replenish the water lost through diarrhea.
OTC treatment options include docusate sodium (Colace) and polyethylene glycol (Miralax) for constipation and loperamide (Imodium A-D) for diarrhea.
If your stomach issues don't resolve, let your healthcare team know. They may adjust your phentermine dose or switch you to a different medication to manage your weight.
7. Sexual side effects
It isn’t common, but there’s a chance that phentermine may cause sexual side effects. These can include erectile dysfunction or changes to your sex drive.
If these side effects become bothersome, talk to your prescriber. They can rule out other possible causes and help you decide on the best course of action.
8. Increased blood pressure and heart rate
Phentermine may cause high blood pressure and a rapid heart rate. This could be due to its effect on your blood vessels. You may notice your heart beating faster when you first start taking phentermine, but it usually isn’t severe and typically goes away as your body adjusts to the medication.
It should be noted that the potential for this side effect isn’t accepted by all researchers. In fact, a recent study suggests the opposite. It found that weight loss aided by phentermine can actually lower blood pressure in people who take it.
High blood pressure is generally symptomless, so it’s hard to gauge the severity without actually taking your blood pressure. If this is a concern for you, it’s a good idea to keep an eye on it using an at-home blood pressure monitor. Symptoms of a fast heart rate are easier to recognize and include dizziness, nausea, and feeling out of breath.
Let your healthcare team know if you have a history of heart problems. They’ll help decide if phentermine is safe for you to take. If you feel dizzy or feel your heart beating rapidly while taking phentermine, let your prescriber know. If at any point you have chest pain or trouble breathing, seek emergency care.
9. Heart valve problems and lung disease
Heart valve problems were discovered when phentermine was taken together with fenfluramine or dexfenfluramine for weight loss in the late 1990s. This combination led to damaged and leaky heart valves, which forced the heart to work harder. The medication that caused this issue — known as fen-phen — was taken off the market for this reason.
Fen-phen also caused primary pulmonary hypertension (PPH), a rare and fatal lung disease, in some people.
Both of these side effects are rare when taking phentermine alone. But they can’t be ruled out as a possibility.
Phentermine isn’t recommended if you have a history of heart problems (such as stroke, a heart rhythm condition, or heart failure). Before starting phentermine, make sure your prescriber has your full medical history. This will help them make sure it’s safe for you to take.
Symptoms of heart valve disease include chest pain, feeling out of breath, and dizziness. PPH symptoms include trouble breathing, chest pain, and swollen ankles. If you experience any of these symptoms while taking phentermine, seek medical care right away.
Frequently asked questions
Phentermine may make you feel more alert and energetic, especially when you first start taking it. This is largely due to phentermine’s stimulant effects. This side effect often goes away as your body adjusts to the medication. So it’s still important to eat a balanced diet and exercise regularly to keep your energy levels at their best.
It’s possible that phentermine can cause you to test positive for amphetamines on a urine drug test. But this shouldn’t be an issue as long as you have a valid prescription and are taking it under the supervision of a medical professional. So it’s a good idea to disclose your prescription before testing occurs to avoid a false positive result from becoming a larger issue.
You should only take phentermine and Ozempic (semaglutide) at the same time if your prescriber recommends it. In most cases, it isn’t recommended to take more than one weight-loss medication at a time. But since Ozempic is a medication for Type 2 diabetes, there’s a chance your prescriber may decide to combine it with phentermine if they decide it’s a safe option for you.
Phentermine may make you feel more alert and energetic, especially when you first start taking it. This is largely due to phentermine’s stimulant effects. This side effect often goes away as your body adjusts to the medication. So it’s still important to eat a balanced diet and exercise regularly to keep your energy levels at their best.
It’s possible that phentermine can cause you to test positive for amphetamines on a urine drug test. But this shouldn’t be an issue as long as you have a valid prescription and are taking it under the supervision of a medical professional. So it’s a good idea to disclose your prescription before testing occurs to avoid a false positive result from becoming a larger issue.
You should only take phentermine and Ozempic (semaglutide) at the same time if your prescriber recommends it. In most cases, it isn’t recommended to take more than one weight-loss medication at a time. But since Ozempic is a medication for Type 2 diabetes, there’s a chance your prescriber may decide to combine it with phentermine if they decide it’s a safe option for you.
The bottom line
Phentermine (Adipex P, Lomaira) is an oral weight loss medication. Its side effects can vary from person to person. But the most common phentermine side effects are dry mouth, insomnia, and headache. Tremor, constipation or diarrhea, and an increased heart rate are also possible.
More serious phentermine side effects, such as mood or behavioral changes, heart valve disease, and high blood pressure in the lungs, are extremely rare. Phentermine isn’t recommended if you have heart problems or a history of substance misuse. If you’re interested in starting phentermine, talk to your prescriber first. They can help decide if it’s a safe and appropriate medication for you.
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References
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