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Phentermine side effects and how to avoid them

Phentermine side effects include Rapid or irregular heartbeat, elevated blood pressure, and restlessness
Keith Gardner, RPh, medical writer and reviewer for SingleCare
By | Mar. 1, 2023
Plate with Rx pills and body tape measure: Phentermine side effects

Key takeaways

  • Common side effects of phentermine include rapid or irregular heartbeat, elevated blood pressure, restlessness, dizziness, and insomnia, among others.

  • Serious side effects of phentermine can include decreased blood flow to the heart, pulmonary hypertension, heart failure, psychosis, and risk of physical or psychological dependence.

  • Phentermine, which is FDA-approved for short-term weight loss, is a Schedule IV controlled substance, indicating a potential for abuse and dependence, and FDA for short-term use in weight loss.

  • Phentermine should only be used by those with a BMI of 30 or greater (or over 27 with certain medical conditions). To minimize side effects, patients should avoid alcohol and follow their healthcare provider’s directions closely.

In this article:

Being overweight or obese is a common and major public health problem in the United States. Both conditions are associated with an increased risk of certain types of cancer, Type 2 diabetes, cardiovascular disease, and other serious conditions that contribute to a reduced quality of life. When a modified calorie diet and exercise alone fail in weight-loss attempts, healthcare providers may suggest adding a weight-loss prescription medicine. 

Phentermine hydrochloride is a prescription-only appetite suppressant, also known as an anorectic, approved by the U.S. Food and Drug Administration (FDA) for short-term use for weight loss. It works by stimulating specific chemicals in the brain that tell the body it is full. Phentermine is a Schedule IV controlled substance, which means it may have the potential for abuse leading to psychological or physical dependence. The most common brand names for phentermine are Adipex-P and Lomaira.

As with any medication, it is important to be aware of adverse effects and drug interactions before beginning therapy. The following information highlights potential phentermine side effects and how to avoid them.

Related: Phentermine alternatives: What can I take instead of phentermine?

Common side effects of phentermine

The most common side effects of phentermine include:

  • Rapid or irregular heartbeat
  • Elevated blood pressure
  • Restlessness
  • Dizziness
  • Insomnia
  • Feelings of euphoria
  • Feelings of dissatisfaction or anxiety (dysphoria)
  • Tremor
  • Headache
  • Dry mouth
  • Unpleasant taste
  • Diarrhea
  • Constipation
  • Itching
  • Impotence
  • Libido changes

Serious side effects of phentermine

The following list includes rare but serious side effects of phentermine:

  • Decreased blood flow to the heart (cardiac ischemia)
  • Rapid beating of the heart (tachycardia)
  • High blood pressure
  • Pulmonary hypertension
  • Heart failure
  • Psychosis
  • Physical or psychological dependence
  • Withdrawal symptoms if abruptly discontinued

RELATED: Is phentermine for weight loss safe?

Phentermine and primary pulmonary hypertension

Weight-control drugs, including phentermine, have been linked to a rare but serious, adverse effect known as primary pulmonary hypertension (PPH). PPH is high blood pressure in the arteries of the lungs. Symptoms of PPH may include:

  • Blue lips and skin 
  • Chest pressure or pain
  • Dizziness or fainting spells 
  • Rapid pulse or pounding heartbeat
  • Fatigue
  • Shortness of breath (dyspnea)

Phentermine and valvular heart disease

There have been rare reports of valvular heart disease in people who have taken phentermine, particularly when paired with fenfluramine, a formerly popular combination drug called “phen-fen” that was discontinued in 1997. Valvular heart disease involves damage to one or more of the heart valves. Symptoms of valvular heart disease may include:

  • Chest pain
  • Palpitations caused by irregular heartbeats
  • Fatigue
  • Dizziness
  • Low or high blood pressure
  • Shortness of breath

Phentermine and risk of abuse

Phentermine is a Schedule IV controlled substance, meaning it carries a risk of abuse and dependence. It is chemically and pharmacologically similar to amphetamines and other stimulant drugs that are subject to abuse. Because it is a stimulant, it can be abused to create a high, particularly when taken in large doses. Phentermine is considered safe when taken as prescribed by a healthcare professional.

How soon do phentermine side effects start?

Many of the most commonly experienced side effects of phentermine occur when initiating therapy but should begin to lessen once the body adjusts to the drug. Less common but more serious side effects, including heart problems, such as primary pulmonary hypertension or valvular heart disease, are typically associated with longer-term use or taking higher than prescribed doses. 

How long do phentermine side effects last?

The recommended duration of the therapy with phentermine is three to six weeks, with a maximum of three months, and the more commonly experienced side effects should end when the medication is stopped, if not sooner as the body adjusts to the medicine.

What are the long-term side effects of phentermine?

Although the FDA-approved use of phentermine is for no longer than three months, there is clinical evidence to support that the use of phentermine for longer-term therapy may be safe and effective. The study found that those who used phentermine for longer than three months did not have increased cardiovascular adverse effects such as high blood pressure or an increased risk of heart attack, stroke, or death. 

Phentermine contraindications

You should not use phentermine if you have any of the following conditions:

  • History of cardiovascular disease (e.g., coronary artery disease, stroke, arrhythmias, congestive heart failure, uncontrolled hypertension)
  • Within 14 days of taking a monoamine oxidase inhibitor 
  • Hyperthyroidism
  • Glaucoma
  • Agitated state
  • History of drug abuse
  • Are pregnant, trying to become pregnant, or breastfeeding
  • Have a known allergy to any active or inactive ingredients 

Phentermine warnings

Children and adolescents

The safety and effectiveness of phentermine in pediatric patients have not been established. Because obesity in children is considered a chronic condition requiring longer-term treatment, the use of phentermine, which is only approved for short-term therapy, is not recommended.

Seniors

Dose selection of phentermine in older adults should start at the low end of the dosing range due to the greater likelihood of decreased liver, kidney, or cardiac function and of concomitant disease or other drug therapy. Phentermine is mainly excreted by the kidney, and the risk of side effects may be particularly greater in older patients who have decreased kidney function.

Pregnancy and breastfeeding

Phentermine is contraindicated during pregnancy because weight loss is not recommended for pregnant women and may result in fetal harm. It is not known if phentermine is present in human milk; therefore, it is not recommended to breastfeed while taking the medication. A woman’s healthcare provider is the best source of information for medical advice when managing weight-loss treatment while breastfeeding.

RELATED: What medications are safe to take during pregnancy?

Overdose

Management of phentermine overdose is largely symptomatic. If an overdose is suspected, a healthcare professional should be contacted immediately. Commonly reported adverse reactions associated with phentermine overdose include: 

  • Restlessness
  • Tremor
  • Overactive or over-responsive bodily reflexes (hyperreflexia)
  • Rapid respiration
  • Confusion
  • Aggressive behavior
  • Hallucinations
  • Panic attacks
  • Cardiovascular effects include: 
    • Irregular heartbeat
    • High or low blood pressure
    • Failure of blood circulation
  • Gastrointestinal symptoms include: 
    • Nausea
    • Vomiting
    • Diarrhea
    • Abdominal cramps

Withdrawal symptoms

Long-term use of phentermine may cause physical and psychological dependence. Abrupt discontinuation after prolonged use may result in withdrawal symptoms including: 

  • Excessive drowsiness
  • Fatigue
  • Tremors
  • Depression

Effect on the ability to engage in potentially hazardous tasks

Taking phentermine may impair the ability to engage in potentially hazardous activities by slowing motor skills and affecting vision. Do not drive a car or operate heavy machinery until you know how phentermine affects you. 

Phentermine interactions

The following have the potential to cause drug interactions when taken at the same time as phentermine:

Monoamine oxidase inhibitors

Use of phentermine is contraindicated within 14 days of taking a monoamine oxidase inhibitor (MAOI) such as isocarboxazid, linezolid, methylene blue injection, phenelzine, rasagiline, selegiline, or tranylcypromine.

Alcohol

Concomitant use of alcohol with phentermine may result in an adverse drug reaction more severe than taking phentermine alone. For example, phentermine side effects can include heart-related symptoms like increased heart rate and high blood pressure. These side effects can worsen when mixing alcohol and phentermine.

RELATED: Is it safe to mix phentermine and caffeine?

Insulin and antidiabetic medications

Phentermine may affect blood sugar levels in people with diabetes mellitus. A reduction in insulin or oral diabetes medications may be required.

Adrenergic neuron-blocking medications

Adrenergic neuron-blocking drugs, although rarely used anymore, may be prescribed to treat high blood pressure or psychotic symptoms. Phentermine may decrease the blood pressure-lowering effect of these drugs.

How to avoid phentermine side effects

Phentermine should only be used by those with a body mass index (BMI) of 30 or greater, or a BMI of over 27 for those with other medical conditions such as controlled hypertension, high cholesterol, or diabetes. The following steps will help reduce the likelihood of experiencing side effects: 

1. Talk to your healthcare professional about your health history

Before starting phentermine, tell your healthcare professional if you have, or have had, any of the following conditions:

  • Abnormal heart rhythms
  • Arteriosclerosis (hardening of the arteries)
  • Congestive heart failure
  • Drug abuse or dependence
  • Heart or blood vessel disease 
  • High blood pressure
  • Hyperthyroidism 
  • Stroke.
  • Heart valve disease 
  • Pulmonary hypertension
  • Kidney disease

2. Give your provider a full list of the medications you’re taking

Tell your healthcare professional about all of the medications you are currently taking, including prescription drugs, over-the-counter medicines, vitamins, and herbal supplements.

3. Avoid alcohol

Alcohol may increase the risk of serious interactions and side effects, such as chest pain and raised blood pressure, and can reduce the effectiveness of phentermine.

4.Follow directions

Take phentermine exactly as directed by your healthcare professional. Do not take more than the prescribed dosage amount, as this may increase the risk of side effects.

How to treat side effects of phentermine

  • Unless otherwise directed by your healthcare professional, phentermine should be taken first thing in the morning on an empty stomach, allowing it to be effective throughout the day and making it easier to fall asleep at night.
  • It is important to stay hydrated when using phentermine. Because it may cause dry mouth, dehydration may make it worse. Try to drink 8 to 12 glasses of water a day to stay hydrated.
  • Contact your healthcare professional if you experience any of the following:
    • Any decrease in the amount of exercise you are normally able to tolerate
    • Accidental overdose
    • Increase in blood pressure
    • Any side effects that bother you or do not go away