What to Do About Them

If you have obesity or are very overweight, you may be interested in learning more about phentermine. It’s a generic prescription drug that’s used to aid weight loss in certain adults. Your doctor may recommend it if you:

Phentermine is used together with diet and exercise. It’s a short-term treatment that you’ll likely take for just a few weeks.

Phentermine comes in three oral forms (which means you take them by mouth):

  • a capsule that you swallow
  • a tablet that you swallow
  • an orally disintegrating tablet that you dissolve on your tongue

Some of these forms come in brand-name versions: Adipex-P capsules, Adipex-P tablets, and Lomaira tablets.

This article describes phentermine’s side effects. For more information about phentermine capsules, including details about its uses, see this in-depth article. Your doctor can tell you more about the other forms of this drug.

It is unclear which of phentermine’s side effects are more common.

The Food and Drug Administration (FDA) approved phentermine for use at a time when the drug approval process was different from what it is now.

At that time, drug studies were not as detailed as they are today. So studies on phentermine didn’t report the frequency of specific side effects. But information on side effects was collected after the drug became available for public use. The sections just below list the mild and serious side effects phentermine is known to cause.

To find out which phentermine side effects you might be more likely to have, ask your doctor. They can discuss some of the side effects their other patients may have had from taking this drug.

Examples of mild side effects that have been reported with phentermine include:

* To learn more about this side effect, see the “Side effects explained” section below.
† An allergic reaction is possible after taking phentermine. But it’s not clear whether this side effect occurred in studies.

In most cases, these side effects should be temporary. And some may be easily managed. But if you have any symptoms that are ongoing or bother you, talk with your doctor or pharmacist. And do not stop taking phentermine unless your doctor recommends it.

Phentermine may cause mild side effects other than the ones listed above. For details, see the prescribing information for the tablet, orally disintegrating tablet, and capsule forms of phentermine.

Note: After the Food and Drug Administration (FDA) approves a drug, it tracks side effects of the medication. If you’d like to notify the FDA about a side effect you’ve had with phentermine, visit MedWatch.

Serious side effects that have been reported with phentermine include:

* To learn more about this side effect, see the “Side effects explained” section below.
† An allergic reaction is possible after taking phentermine. But it’s not clear whether this side effect occurred in studies.

If you develop serious side effects while taking phentermine, call your doctor right away. If the side effects seem life threatening or you think you’re having a medical emergency, immediately call 911 or your local emergency number.

Yes, phentermine can cause sexual side effects in both females and males.*

Phentermine can increase or decrease libido (sex drive) in females taking this drug.

Males who take phentermine can also experience changes in libido. And the drug may cause erectile dysfunction (ED).

It’s important to note that you’ll only take phentermine for a few weeks. Sexual side effects usually get better after you stop taking phentermine. For example, any changes to your sex drive should return to normal after you stop taking the drug.

If you’re concerned about sexual side effects from taking phentermine, talk with your doctor.

* In this article, we use the terms “male” and “female” to refer to someone’s sex assigned at birth. For information about the difference between sex and gender, see this article.

Get answers to some frequently asked questions about phentermine’s side effects.

Can phentermine cause long-term side effects?

It might, but this is rare. Most of phentermine’s side effects get better in a few days or weeks, or soon after you stop taking the medication.

But there have been some rare reports of heart valve damage in people taking phentermine. This could lead to long-term heart problems. To learn more about this side effect, see “Phentermine’s side effects explained” below.

Long-term use of phentermine is not recommended because this could increase your risk of dependence on the medication. With dependence, you rely on a drug to function as usual, either physically or mentally.

Long-term use of the drug can also cause serious side effects such as personality changes and psychosis (loss of contact with reality). You’ll usually take phentermine for just a few weeks to avoid these problems.

If you’re concerned about the risk of long-term side effects with phentermine, talk with your doctor.

Will my risk of side effects increase if I take a higher dose of phentermine (37.5 mg)?

Yes, it might. As with many medications, higher doses of phentermine, such as 37.5 milligrams (mg), may be more likely to cause side effects than lower doses.

Keep in mind that many factors can affect your risk of side effects. These include other conditions you may have and other medications you may take.

If you’re concerned about the risk of side effects with the phentermine dosage you’ve been prescribed, talk with your doctor.

Also talk with them if you feel the dosage you’ve been prescribed isn’t working for you. Do not increase your dosage unless your doctor recommends you do so. If phentermine stops working for you, they’ll likely recommend stopping treatment rather than increasing your dosage. Your doctor can discuss alternative methods to help with weight loss.

Does phentermine cause any psychological side effects?

Yes, it may. For example, phentermine can often cause mood changes, such as feelings of intense excitement or happiness. It can also cause feelings of unease and restlessness.

Misuse of phentermine can result in more serious psychological effects. (Misuse refers to taking a drug in way or for a use that’s not prescribed by a doctor.) Examples include irritability, personality changes, and psychosis (loss of contact with reality).

Misuse can also lead to psychological dependence on the drug. To learn more about this, see “Phentermine and misuse” below.

If you’re concerned about psychological side effects with phentermine, talk with your doctor.

If I stop taking phentermine, will I develop withdrawal symptoms?

Not usually. You’ll likely take phentermine for only a few weeks. Stopping phentermine after short-term use doesn’t usually cause withdrawal symptoms.

Phentermine reduces your appetite. So after stopping treatment, your appetite may increase again. This could lead to weight gain. Talk with your doctor about how to maintain changes in your eating habits after you stop taking phentermine. This can help you avoid regaining any weight you’ve lost.

Note that misuse of phentermine can lead to dependence on the drug. With dependence, you rely on a drug to function as usual, either physically or mentally. Suddenly stopping phentermine after taking high doses over a long period can cause withdrawal symptoms. These include depression and extreme fatigue (low energy).

To learn more about phentermine misuse, see “Phentermine and misuse” below.

Learn more about some of the side effects phentermine may cause.

Constipation

You may have constipation while taking phentermine. But it’s not known how often this side effect occurs. Note that phentermine can also cause diarrhea.

If you have constipation, it’s usually mild to moderate and easily managed.

What might help

You may be able to ease constipation by:

  • drinking more water
  • getting exercise regularly
  • eating more fiber-rich foods, such as fresh or dried fruit, fresh vegetables, beans, and whole grains

These measures can also help with weight loss. And keep in mind that phentermine is a weight-loss aid.

If you have constipation that doesn’t improve with lifestyle measures such as those suggested above, talk with your doctor or pharmacist. They can give you more dietary advice. They may also recommend taking a laxative medication.

Headaches

You may have headaches while taking phentermine. It’s not known how often this side effect occurred in studies.

If you have headaches, they’ll likely be mild and easily managed.

What might help

Remedies you can try at home to relieve headaches include:

  • drinking more water
  • getting plenty of fresh air
  • gently massaging your temples
  • taking time to rest and relax

If you have bothersome headaches, it’s usually safe to take over-the-counter pain relievers with phentermine. Examples include Tylenol (acetaminophen) and Advil (ibuprofen). But avoid taking pain medications that also contain caffeine. Phentermine and caffeine are both stimulants. Taking them together could increase side effects such as restlessness and trouble sleeping.

You can ask your pharmacist to recommend a suitable pain reliever.

Heart-related side effects

Phentermine can sometimes cause heart-related side effects, which tend to be mild to moderate. It’s not known how often these side effects occur with phentermine. Examples include:

if you have heart disease or high blood pressure that’s not well managed, these side effects could worsen your condition. In this case, your doctor will likely not prescribe phentermine.

Phentermine can also cause a serious side effect called valvular heart disease, which involves damage to your heart valves. The damage stops the valves from closing properly, so they may leak. This can affect the way blood flows through your heart and around your body. This side effect is rare.

Symptoms of valvular heart disease include:

What might help

While you’re taking phentermine, your doctor may monitor your heart rate and blood pressure.

If you have symptoms of heart-related side effects, especially valvular heart disease, during treatment, contact your doctor right away.

Pulmonary arterial hypertension

Though rare, a serious side effect called pulmonary arterial hypertension (PAH) may occur with phentermine. (This condition is sometimes referred to as primary pulmonary hypertension.) Most people who reported this side effect were also taking Fintepla (fenfluramine) or dexfenfluramine. Like phentermine, these are stimulant drugs.

PAH is a potentially life threatening condition in which you have high blood pressure in the arteries supplying your lungs. This can cause breathing problems and heart failure.

Symptoms of PAH include:

  • shortness of breath
  • chest pain
  • fainting
  • swollen ankles and feet

What might help

If you have symptoms of PAH while taking phentermine, contact your doctor right away. They’ll order tests to determine whether you have this condition.

Allergic reaction

Like most drugs, phentermine can cause an allergic reaction in some people. But it’s not clear whether this side effect occurred in studies.

Symptoms can be mild to serious and can include:

  • skin rash
  • itchiness
  • flushing (temporary warmth, redness, or deepening of skin color)
  • swelling under your skin, usually in your eyelids, lips, hands, or feet
  • swelling of your mouth, tongue, or throat, which can make it hard to breathe

What might help

If you have mild symptoms of an allergic reaction, such as a mild rash, call your doctor right away. They may suggest a treatment to manage your symptoms. Examples include:

  • an antihistamine you take by mouth, such as Benadryl (diphenhydramine)
  • a product you apply to your skin, such as hydrocortisone cream

If your doctor confirms you’ve had a mild allergic reaction to phentermine, they’ll decide if you should continue taking it.

If you have symptoms of a severe allergic reaction, such as swelling or trouble breathing, call 911 or your local emergency number right away. These symptoms could be life threatening and require immediate medical care.

If your doctor confirms you’ve had a serious allergic reaction to phentermine, they may prescribe a different treatment.

Keeping track of side effects

During your phentermine treatment, consider taking notes on any side effects you’re having. You can then share this information with your doctor. This is especially helpful when you first start taking new drugs or using a combination of treatments.

Your side effect notes can include things such as:

  • what dose of the drug you were taking when you had the side effect
  • how soon you had the side effect after starting that dose
  • what your symptoms were
  • how it affected your daily activities
  • what other medications you were taking
  • any other information you feel is important

Keeping notes and sharing them with your doctor will help them learn more about how phentermine affects you. They can then use this information to adjust your treatment plan if needed.

Phentermine may not be right for you if you have certain medical conditions. These are known as drug-condition interactions. Other factors may also affect whether phentermine is a good treatment option for you.

Talk with your doctor about your health history before starting phentermine. Factors to consider include those described below.

Heart disease or stroke. Phentermine can cause heart-related side effects.If you have heart disease, it could make your condition worse. If you’ve had a stroke, it increases your risk of another stroke. In such cases, your doctor will likely not prescribe phentermine. Ask them what other medications are better options for you.

High blood pressure. Phentermine can worsen high blood pressure. If you have high blood pressure that’s not well managed with medication, your doctor will likely not prescribe phentermine. Ask them what other medications are better options for you.

If you have high blood pressure that’s well managed with medication, your doctor will likely check your blood pressure more often than usual while you take phentermine.

Overactive thyroid gland. If you have an overactive thyroid gland, phentermine could make your symptoms worse. Due to this risk, your doctor will likely not prescribe phentermine. Ask them what other medications are better options for you.

Glaucoma. Phentermine can cause eye effects that may worsen glaucoma. If you have glaucoma, your doctor will likely not prescribe phentermine. Ask them what other medications are better options for you.

Allergic reaction. If you’ve had an allergic reaction to phentermine, any of its ingredients, or other stimulant drugs, your doctor will likely not prescribe phentermine. Ask them what other medications are better options for you.

Agitation. Phentermine can cause restlessness, tremors, and excitement, so it could worsen agitation. Agitation is commonly associated with mental health conditions, such as anxiety, bipolar disorder, and schizophrenia. If you have agitation, your doctor will likely not prescribe phentermine. Ask them what other medications are better options for you.

Past drug misuse. Phentermine has a risk of misuse. If you’ve misused drugs in the past, your doctor will likely not prescribe phentermine. Ask them what other medications are better options for you. To learn more about phentermine misuse, see the “Phentermine and misuse” section below.

Kidney problems. Your kidneys remove phentermine from your body. If your kidneys don’t work well, phentermine could build up in your body, increasing your risk of side effects. If you have kidney problems, your doctor may prescribe a phentermine dosage that’s lower than usual.

Diabetes. Phentermine reduces your appetite and helps you lose weight. As a result, your blood sugar levels can decrease. If you have diabetes, your doctor may check your blood sugar more frequently while you take phentermine. If needed, they may reduce the dosage of any diabetes medication that you take.

Alcohol and phentermine

Your doctor will likely recommend that you avoid alcohol while taking phentermine. Drinking alcohol with phentermine could increase your risk of misuse for either drug. It could also increase your risk of certain side effects, such as:

If you drink alcohol, talk with your doctor about how much alcohol, if any, is safe to consume while you’re taking phentermine.

To learn more about phentermine misuse, see “Phentermine and misuse” below.

Pregnancy and breastfeeding while taking phentermine

Phentermine is not safe to take during pregnancy or while breastfeeding.

If you’re pregnant, taking phentermine could harm your developing fetus. As well, weight loss is not recommended during pregnancy, even if you have obesity or are overweight.

If you’re breastfeeding, phentermine could pass into your breast milk. This could cause serious side effects in a child who’s breastfed. Talk with your doctor about other ways to manage your weight while you’re breastfeeding.

Phentermine is related to amphetamine drugs. It’s a controlled substance. Controlled substances have a recognized medical use but are regulated by the United States government due to risk of misuse and dependence.

Misuse refers to taking a drug in way or for a use that’s not prescribed by a doctor, such as taking more than the recommended dosage. Drug misuse can cause side effects, some of which can be serious. Examples include:

Misuse can also lead to drug dependence. With drug dependence, you rely on a drug to function as usual, either mentally or physically. As a result, you may feel unable to stop taking the drug. And you may have withdrawal symptoms if you stop taking it.

If you’ve misused drugs in the past, you may have an increased risk of phentermine misuse. Due to this risk, your doctor will likely not prescribe this medication.

You should only take phentermine as prescribed by your doctor. Do not increase your dosage or take it for longer than recommended. If you have concerns about the risk of misuse from taking this drug, talk with your doctor.

Most of phentermine’s side effects are mild and easily managed. But this medication can also cause some serious side effects. It’s important to take it only as prescribed by your doctor. Your doctor can tell you more about the pros and cons of taking this medication. And they can help you decide if it’s a good option for you.

Here are a few questions that you may want to ask your doctor about this drug:

  • Is it safe and legal to drive while taking phentermine?
  • Can I develop an addiction to phentermine if I only take it for a few weeks?
  • Can phentermine cause heart attacks?
  • How do phentermine’s side effects compare with other treatment options for my condition?

If you have any questions about side effects that phentermine can cause, talk with your doctor.

You can also ask them about Adipex-P and Lomaira, which are brand-name versions of phentermine tablets. Adipex-P also comes as capsules. A generic drug and its brand-name version are expected to have similar side effects because they contain the same active ingredient. (An active ingredient is what makes a drug work.)

To learn more about phentermine, see these articles:

To get information on different conditions and tips for improving your health, subscribe to any of Healthline’s newsletters. You may also want to check out the online communities at Bezzy. It’s a place where people with certain conditions can find support and connect with others.

Disclaimer: Healthline has made every effort to make certain that all information is factually correct, comprehensive, and up to date. However, this article should not be used as a substitute for the knowledge and expertise of a licensed healthcare professional. You should always consult your doctor or another healthcare professional before taking any medication. The drug information contained herein is subject to change and is not intended to cover all possible uses, directions, precautions, warnings, drug interactions, allergic reactions, or adverse effects. The absence of warnings or other information for a given drug does not indicate that the drug or drug combination is safe, effective, or appropriate for all patients or all specific uses.

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