Phentermine is used with a doctor-approved exercise, behavior change, and reduced-calorie diet program to help you lose weight. It is used by certain overweight people, such as those who are obese or have weight-related medical problems. Losing weight and keeping it off can lessen the many health risks that come with obesity, including heart disease, diabetes, high blood pressure, and a shorter life.

It is not known how this medication helps people to lose weight. It may work by decreasing your appetite, increasing the amount of energy used by your body, or by affecting certain parts of the brain. This medication is an appetite suppressant and belongs to a class of drugs called sympathomimetic amines.

Take this medication by mouth as directed by your doctor, usually once a day, 1 hour before breakfast or 1 to 2 hours after breakfast. If needed, your doctor may adjust your dose to take a small dose up to 3 times a day. Carefully follow your doctor's instructions. Taking this medication late in the day may cause trouble sleeping (insomnia).

If you are using sustained-release capsules, the dose is usually taken once a day before breakfast or at least 10 to 14 hours before bedtime. Swallow the medication whole. Do not crush or chew sustained-release capsules. Doing so can release all of the drug at once, increasing the risk of side effects.

If you are using the tablets made to dissolve in the mouth, the dose is usually taken once a day in the morning, with or without food. First, dry your hands before handling the tablet. Place your dose on top of the tongue until it dissolves, then swallow it with or without water.

The dosage and length of treatment are based on your medical condition and response to treatment. Your doctor will adjust the dose to find the best dose for you. Discuss the risks and benefits, and the length of phentermine treatment, with your doctor. To get the most benefit, take this medication regularly, exactly as prescribed, and for as long as directed by your doctor. To help you remember, take it at the same time(s) each day.

If you suddenly stop using this medication, you may have withdrawal symptoms (such as depression, severe tiredness). To help prevent withdrawal, your doctor may lower your dose slowly. Withdrawal is more likely if you have used phentermine for a long time or in high doses. Tell your doctor or pharmacist right away if you have withdrawal.

Though it helps many people, this medication may sometimes cause addiction. This risk may be higher if you have a substance use disorder (such as overuse of or addiction to drugs/alcohol). Do not increase your dose, take it more often, or use it for a longer time than prescribed. Properly stop the medication when so directed.

This medication may stop working well after you have been taking it for a few weeks. Talk with your doctor if this medication stops working well. Do not increase the dose unless directed by your doctor. Your doctor may direct you to stop taking this medication.

Dizziness, dry mouth, difficulty sleeping, irritability, nausea, vomiting, diarrhea, or constipation may occur. If any of these effects last or get worse, tell your doctor or pharmacist promptly.

Remember that this medication has been prescribed because your doctor has judged that the benefit to you is greater than the risk of side effects. Many people using this medication do not have serious side effects.

This medication may raise your blood pressure. Check your blood pressure regularly and tell your doctor if the results are high.

Tell your doctor right away if you have any serious side effects, including:

• fast/irregular/pounding heartbeat
• mental/mood changes (such as agitation, uncontrolled anger, hallucinations, nervousness)
• uncontrolled muscle movements
• change in sexual ability/interest

Stop taking this medication and get medical help right away if you have any very serious side effects, including:

• severe headache
• trouble speaking
• seizure
• weakness on one side of the body
• vision changes (such as blurred vision)

This drug may rarely cause serious (sometimes fatal) lung or heart problems (pulmonary hypertension, heart valve problems). The risk may increase with longer use of this medication and use of this drug along with other appetite-suppressant drugs/herbal products. Stop taking this medication and get medical help right away if you have any very serious side effects, including:

• chest pain
• difficulty breathing with exercise
• decreased ability to exercise
• fainting
• swelling of the legs/ankles/feet

A very serious allergic reaction to this drug is rare. However, get medical help right away if you notice any symptoms of a serious allergic reaction, including:

• rash
• itching/swelling (especially of the face/tongue/throat)
• severe dizziness
• trouble breathing

This is not a complete list of possible side effects. If you notice other effects not listed above, contact your doctor or pharmacist.

In the US - Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088 or at www.fda.gov/medwatch.

In Canada - Call your doctor for medical advice about side effects. You may report side effects to Health Canada at 1-866-234-2345.

Before taking this medication, tell your doctor or pharmacist if you are allergic to it; or to any other sympathomimetic amines (including decongestants such as pseudoephedrine, stimulants such as amphetamine, appetite suppressants such as diethylpropion); or if you have any other allergies. This product may contain inactive ingredients, which can cause allergic reactions or other problems. Talk to your pharmacist for more details.

Before using this medication, tell your doctor or pharmacist your medical history, especially of:

• diabetes
• high blood pressure
• glaucoma
• personal or family history of a substance use disorder (such as overuse of or addiction to drugs/alcohol)
• heart disease (such as chest pain, heart attack, heart murmur, fast/irregular heartbeat, heart valve problems)
• mental/mood problems (such as depression, thoughts of suicide, severe anxiety/agitation)
• high blood pressure in the lungs (pulmonary hypertension)
• stroke
• overactive thyroid (hyperthyroidism)
• kidney disease
• seizures

This drug may make you dizzy or blur your vision. It may also rarely make you drowsy. Alcohol or marijuana (cannabis) can make you more dizzy or drowsy. Do not drive, use machinery, or do anything that needs alertness or clear vision until you can do it safely. Avoid alcoholic beverages. Talk to your doctor if you are using marijuana (cannabis).

If you have diabetes, check your blood sugar regularly as directed and share the results with your doctor. Your doctor may need to adjust your diabetes medication during treatment with this drug.

If you are using the tablets made to dissolve in the mouth, your medication may contain aspartame. If you have phenylketonuria (PKU) or any other condition that requires you to limit/avoid aspartame (or phenylalanine) in your diet, ask your doctor or pharmacist about using this medication safely.

Before having surgery, tell your doctor or dentist about all the products you use (including prescription drugs, nonprescription drugs, and herbal products).

Older adults may be at greater risk for dizziness and high blood pressure while using this drug.

This medication must not be used during pregnancy. It may harm an unborn baby. If you are pregnant or think you may be pregnant, tell your doctor right away.

This drug may pass into breast milk and could have undesirable effects on a nursing infant. Breastfeeding is not recommended while using this drug. Consult your doctor before breastfeeding.

Drug interactions may change how your medications work or increase your risk for serious side effects. This document does not contain all possible drug interactions. Keep a list of all the products you use (including prescription/nonprescription drugs and herbal products) and share it with your doctor and pharmacist. Do not start, stop, or change the dosage of any medicines without your doctor's approval.

This drug should not be used with certain medications because very serious interactions may occur. If you are taking or have taken other appetite-suppressant drugs in the past year (such as diethylpropion, ephedra/ma huang), tell your doctor or pharmacist before starting this medication.

Taking MAO inhibitors with this medication may cause a serious (possibly fatal) drug interaction. Avoid taking MAO inhibitors (isocarboxazid, linezolid, metaxalone, methylene blue, moclobemide, phenelzine, procarbazine, rasagiline, safinamide, selegiline, tranylcypromine) during treatment with this medication. Most MAO inhibitors should also not be taken for two weeks before treatment with this medication. Ask your doctor when to start or stop taking this medication.

Some products that may interact with this drug are:

• high blood pressure medicine (such as guanethidine, methyldopa)
• phenothiazines (such as prochlorperazine, chlorpromazine)
• other stimulants (including amphetamines, methylphenidate, street drugs such as cocaine or MDMA/"ecstasy")

Tell your doctor or pharmacist if you are taking other products that cause drowsiness such as opioid pain or cough relievers (such as codeine, hydrocodone), alcohol, marijuana (cannabis), drugs for sleep or anxiety (such as alprazolam, lorazepam, zolpidem), muscle relaxants (such as carisoprodol, cyclobenzaprine), or antihistamines (such as cetirizine, diphenhydramine).

Check the labels on all your medicines (such as allergy or cough-and-cold products) because they may contain ingredients that cause drowsiness. Ask your pharmacist about using those products safely.

Some products have ingredients that could raise your heart rate or blood pressure. Tell your pharmacist what products you are using, and ask how to use them safely (especially cough-and-cold products or diet aids).

Caffeine can increase the side effects of this medication. Avoid drinking large amounts of beverages containing caffeine (coffee, tea, colas) or eating large amounts of chocolate.

This medication may interfere with certain medical/lab tests (such as brain scan for Parkinson's disease), possibly causing false test results. Make sure lab personnel and all your doctors know you use this drug.

If someone has overdosed and has serious symptoms such as passing out or trouble breathing, call 911. Otherwise, call a poison control center right away. US residents can call 1-800-222-1222. Canada residents can call 1-844-764-7669. Symptoms of overdose may include: rapid breathing, unusual restlessness, fast/slow/irregular heartbeat, chest pain, hallucinations, seizures, loss of consciousness.

Appetite suppressants should not be used in place of proper diet. For best results, this drug must be used along with a doctor-approved diet and exercise program.

Do not share this medication with others. Sharing it is against the law.

Lab and/or medical tests (such as blood pressure, heart tests, kidney tests) should be done while you are taking this medication. Keep all medical and lab appointments. Consult your doctor for more details.

If you miss a dose, take it as soon as you remember. If it is near the time of the next dose or late in the evening, skip the missed dose. Take your next dose at the regular time. Do not double the dose to catch up.

Store at room temperature away from light and moisture. Do not store in the bathroom. Keep all medications away from children and pets.

Do not flush medications down the toilet or pour them into a drain unless instructed to do so. Properly discard this product when it is expired or no longer needed. Consult your pharmacist or local waste disposal company.