Phentermine Side Effects You Should Know

Obesity is a chronic condition with well-known health risks. The initial approach to treat obesity is a lifestyle change such as eating a healthy diet and maintaining a consistent exercise routine. However, diet and exercise alone are sometimes not enough to achieve the desired results. In that case, some people may benefit from adding a weight loss medication to their lifestyle changes to treat obesity, especially for people with risk factors such as high blood pressure, diabetes, and high cholesterol. Like most medications, weight loss drugs come with risks such as side effects and drug interactions. Please continue reading to learn more about the side effects of phentermine, a medication commonly prescribed to help with weight management.

What is phentermine?

Phentermine is used as an appetite suppressant; it is a schedule IV prescription medication; however, the rate of misuse seems to be low.

Phentermine is classified as a sympathomimetic with structural similarity to amphetamine. Phentermine is used to treat obesity alongside a low-calorie diet and exercise, especially for people with high risks for stroke and heart diseases due to pre-existing high blood pressure, diabetes, and high cholesterol.

There is limited data on the exact mechanism of phentermine. Similar to amphetamine, phentermine is believed to increase the release of the natural brain chemicals, dopamine and norepinephrine, in the central nervous system. This mechanism stimulates the fight or flight mode, thus, your blood pressure and heart rate are elevated while the appetite is suppressed. Another way that phentermine decreases appetite is through direct stimulation of the satiety center in the brain.

Who can take phentermine?

Phentermine is approved for adults and children 17 years and older. The safety and effectiveness of this drug in children 16 and younger has not been established.

Phentermine is approved to be used short-term as part of weight loss management along with a reduced-calorie diet and lifestyle modifications. You should talk to your doctor if you have not been able to lose weight through diet and exercise alone and you meet the below criteria:

A body mass index (BMI) of 30 kg/m2 (66lbs) or higher (this is classified as obesity).
A body mass index (BMI) of 27 kg/m2 (59 lbs) or higher (this is classified as overweight) with at least one of the weight-related risk factors like high blood pressure that is controlled, high cholesterol, or diabetes.

How long can you stay on phentermine?

Phentermine is for short-term use; it is approved to be used for up to 12 weeks. Most people take it for 3-6 weeks. You should let your doctor know if you don’t experience any effect from phentermine by week 4. Weight loss management depends on a lot of factors. Some people may take longer to start experiencing weight loss; therefore, sometimes, providers may extend the duration of phentermine beyond 12 weeks. Phentermine and topiramate are the active ingredients of Qsymia; this combination pill is approved for long-term use.

You should use phentermine exactly as prescribed by your healthcare provider. Read the prescription label carefully. Follow your doctor’s dosing instructions. Use this medicine only for the indication prescribed. Do not take phentermine at a higher dose, more frequently, or for longer than advised. Phentermine is a Schedule IV controlled substance, which is defined as a drug with low potential for abuse and low risk of dependence.

What is the dose of phentermine?

Phentermine comes in the following dosage forms:

Oral tablets
- Tablets (brand name: Lomaira) to be taken at a dose of 8 mg three times a day 30 minutes before meals.
- Tablets (generic phentermine, Adipex P) to be taken at a dose of 18.75 to 37.5 mg once a day, 30 minutes before breakfast, or 1-2 hours after breakfast; another dosing option is 18.75 mg by mouth twice daily.
Oral capsules
- Adipex-P and other generic 37.5 mg capsules: take 37.5 mg by mouth once a day, 30 minutes before breakfast or 1-2 hours after breakfast;
- Generic 15 mg and 30 mg capsules: take 15 to 30 mg by mouth once per day 2 hours after breakfast.
- Extended-release capsules to be taken at a dose of 15 mg to 30 mg once a day, 1 hour before breakfast or 1-2 hours after breakfast, at least 10-14 hours before bedtime.
Orally disintegrating tablets (ODT)
- Orally disintegrating tablets to be taken at a dose of 15 mg, 30 mg, or 37.5 mg. Dissolve one tablet under the tongue once a day in the morning.

How to use phentermine?

Swallow the tablets with a glass of water. Make sure you swallow the extended-release capsules of phentermine whole. Do not crush, break, or chew them. Handle the orally disintegrating tablet with dry hands and place it on your tongue immediately where it will melt quickly.

This medication is typically taken before breakfast, or 1 to 2 hours after breakfast; follow your doctor’s instructions on how to take it.

If you miss a dose of phentermine, take it as soon as you remember. However, if it is almost time for the next dose, skip the missed dose and take the next dose at the scheduled time. Do not take extra medicine to make up for a missed dose.

Store phentermine at room temperature and keep all medicines safely out of reach of children and pets.

Do not stop taking phentermine without talking to your doctor first. Stopping this medication abruptly can lead to unpleasant withdrawal symptoms like tremors, fatigue, drowsiness, and depression. Your doctor will tell you how to reduce the dose gradually over a few weeks.

What are the side effects of phentermine?

Common adverse effects of phentermine include fast heart rate, nervousness, sleeplessness (trouble sleeping), constipation, diarrhea, vomiting, stomach pain, dry mouth, and an unpleasant taste in the mouth. Let your doctor know if these side effects do not go away after a few days. Call your doctor for medical advice about side effects not listed in your medication guide.

Can phentermine cause serious side effects?

Tell your doctor immediately or seek emergency medical attention if you experience:

Heart palpitations
Increased blood pressure or uncontrolled high blood pressure
Shortness of breath
Chest pain
Skin rash, hives, swelling, itching
Swelling of the face, lips, tongue, or throat
Swelling of the ankles or feet
Reduced exercise tolerance (problems doing exercise you could previously do)
Tremor
Low blood sugar

You may report side effects that are unexpected or unusual to the FDA at 1-800-FDA-1088.

In case of an overdose, call the poison control helpline at 1-800-222-1222 or seek emergency medical help at your nearest emergency room. Overdose symptoms may include blurred vision, confusion, hallucinations, restlessness, stomach cramps, nausea, vomiting, diarrhea, fatigue, depression, weak pulse, irregular heartbeat, seizures, and slowed or stopped breathing.

What should I avoid when taking phentermine?

Taking phentermine with other medicines can lead to a potentially dangerous drug interaction. The following medications are known to affect phentermine or interact with it:

Selective-serotonin reuptake inhibitors such as fluoxetine (Prozac), paroxetine (Paxil), fluvoxamine (Luvox), and sertraline (Zoloft).
Other diet medications.

Tell your doctor if you are taking medicines called monoamine oxidase (MAO) inhibitors (for example, phenelzine (Nardil), isocarboxazid (Marplan), tranylcypromine (Parnate), and selegiline (Eldepryl, Zelapar, Emsam), or if you have taken them in the past two weeks. Do not start phentermine as it can lead to dangerous side effects.

The above are not all possible interactions of phentermine. Give your doctor and pharmacist a complete list of your current medicines, including prescription, over-the-counter medicines, dietary supplements, other diet pills, and herbal products to avoid potentially dangerous drug interactions.

What precautions should I take while on phentermine?

Call your doctor if you become pregnant while on phentermine. This medicine may be harmful to the fetus.

If you are an older adult over 65 years of age, discuss the risks versus benefits of taking phentermine with your doctor. There may be safer alternatives available.

Phentermine can cause drowsiness and dizziness. Do not drive, operate heavy machinery, or do anything that requires your full focus until you know how phentermine affects you. Avoid alcohol while on phentermine as it can make these side effects worse.

If you have diabetes and take insulin, changes in your blood sugar levels may occur while on phentermine. Your doctor may reduce the insulin dose while you are on phentermine. Talk to your doctor about adjusting your diabetes medication dose.

Why should you not take phentermine?

Phentermine is one of the most commonly prescribed weight loss medications, but it may not be safe for people with certain health problems. You should not take phentermine if you are pregnant. Be sure to tell your doctor if you become pregnant while on this medicine.

Additionally, phentermine may not be a good choice if you have high blood pressure, cardiovascular disease, heart problems like coronary artery disease, thyroid disease (overactive thyroid gland), glaucoma, diabetes, or a history of drug abuse.

Historically, “fen-phen” (fenfluramine/phentermine) has caused heart valve diseases, and a rare fatal lung disorder called pulmonary hypertension, causing a high pressure in the blood vessels leading from the heart to the lungs. It is probably even rarer with phentermine by itself; however, you should still be cautious and inform your doctor immediately if you experience shortness of breath, chest pain, and swelling.

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