Phentermine Side Effects: Can It Cause Anxiety?

Key Takeaways

• Phentermine is a stimulant medication similar to amphetamines, and it's used to help people with obesity lose weight.
• Common side effects include dry mouth, headache, nausea, and insomnia. Severe side effects may include hypertension, restlessness, and heart-related issues.
• Some people may experience anxiety while taking phentermine due to its stimulant effects.

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Phentermine is one of the oldest weight loss drugs around. While it's a safe and effective medication for people with obesity, that doesn't mean it's risk-free.

As a stimulant, phentermine affects the central nervous system — so while it can suppress appetite, it can also lead to unwanted side effects. Some people experience dry mouth and headaches, while others may get more concerning symptoms, such as increased heart rate, restlessness, or even anxiety. For people prone to anxiety or those sensitive to stimulants, the medication may make nervousness or jitteriness worse. 

Here's everything you need to know about phentermine's potential side effects, when to seek medical attention, and how to manage symptoms.

What Is Phentermine?

Phentermine is a prescription appetite suppressant used for short-term weight management in people with obesity and a body mass index (BMI) of 30 or more. It is typically prescribed with a reduced-calorie diet and regular exercise to help with weight loss. Phentermine belongs to a class of drugs known as sympathomimetic amines, which stimulate the central nervous system to increase metabolism and decrease appetite. It is chemically similar to amphetamines. 

Approved by the U.S. Food and Drug Administration (FDA) in 1959, phentermine is available under different brand names, including Adipex-P, Lomaira, and Qsymia (extended-release capsules that contain a combination of phentermine and topiramate). Because it's a stimulant, it is classified as a Schedule 4 controlled substance, meaning it has a potential for abuse and dependence. It is usually prescribed short-term — for just a few weeks or up to three months — to prevent the risk of gaining a tolerance to it and to minimize adverse effects.

Phentermine primarily works by increasing the release of neurotransmitters, such as norepinephrine, dopamine, and serotonin. These chemicals signal the brain to suppress hunger and boost energy, making it easier to stick to a calorie-restricted diet. However, this mechanism of action can also lead to side effects, including anxiety and restlessness. 1

What Are Phentermine Side Effects?

Like any medication, phentermine has potential side effects, which can vary in severity. While some people tolerate the medication well, others may experience uncomfortable symptoms. It's worth noting that males and females can experience the same side effects while taking phentermine.

1. Common Side Effects

Some of the most common side effects of phentermine include:

• Dry mouth
• Bad taste in the mouth
• Nervousness, sweating, and palpitations 
• Headache
• Nausea
• Constipation

These side effects are usually mild and may diminish as the body adjusts to the medication. However, if they persist or worsen, you should schedule a visit with your healthcare provider.

2. Severe Side Effects

Although less common, phentermine can also cause more serious side effects, including the following. Note that these severe side effects require immediate medical attention.  ^{2 3}

• Hypertension (high blood pressure)
• Tremors
• Insomnia
• Erectile dysfunction in men
• Valvular heart disease
• Primary pulmonary hypertension (a rare but serious lung condition)
• Chest pain
• Difficulty breathing
• Swelling in the legs
• Irregular heartbeat

Can Phentermine Cause Anxiety?

Phentermine’s stimulant properties can lead to increased heart rate, irritability, nervousness, excitability, insomnia, sweating, and fast heartbeat — all factors that may contribute to anxiety in some people. And research shows that these are common symptoms. 

Phentermine affects neurotransmitters like norepinephrine, which plays a role in the body’s fight-or-flight response. Elevated norepinephrine levels can cause symptoms such as nervousness and jitteriness in people who are sensitive to stimulants or have a history of anxiety disorders. What's more, research shows that phentermine might lead to depression.

If you have a history of anxiety or related disorders, talk with your doctor before starting phentermine. If you're noticing these common side effects on phentermine and it's worsening your anxiety, speak to your doctor about adjusting the dosage or switching to an alternative weight loss medication. ^{4 5}

When to Seek Medical Attention for Phentermine Side Effects

While many of phentermine’s side effects are mild and temporary, certain symptoms warrant immediate medical attention. If you experience severe symptoms such as chest pain, difficulty breathing, extreme mood changes, or signs of an allergic reaction (swelling, rash, severe dizziness), see your healthcare professional as soon as possible.

Other serious side effects that require a prompt doctor's visit include persistent high blood pressure, irregular heartbeat, or decreased exercise tolerance. Ignoring severe side effects can lead to serious health complications.

Tips to Avoid Phentermine Side Effects

To minimize the risk of side effects, consider the following precautions:

• Follow prescribed directions: Take phentermine exactly as directed by your healthcare provider.
• Provide a complete list of medications: Tell your doctor of any medications or supplements you are taking before starting phentermine to avoid interactions.
• Avoid drinking alcohol: Alcohol can exacerbate certain side effects, such as dizziness and increased heart rate while you're on phentermine.
• Discuss your medical history: If you have a history of heart disease, mental health disorders, or high blood pressure, consult your doctor before starting phentermine.

Managing Phentermine Side Effects

If side effects occur, they can often be managed by making some adjustments. Consider these strategies to help:

• Take phentermine in the morning: This reduces the risk of insomnia by allowing the stimulant effects to wear off before bedtime.
• Stay hydrated: Drink at least 8 glasses of water daily to help with common side effects, such as dry mouth and digestive issues.
• Contact your doctor if you experience:
  
  • Reduced exercise tolerance
  • Signs of overdose (severe restlessness, confusion, hallucinations)
  • Persistently high blood pressure
  • Lingering or worsening side effects

Who Should Avoid Phentermine?

While phentermine is an effective weight-loss medication, it's not the best choice for everyone looking to lose weight. Certain medical conditions and risk factors make it unsafe, increasing the likelihood of severe side effects or complications. The following people should avoid taking phentermine: ^{6 7 8 9 10}

1. People with an Overactive Thyroid (Hyperthyroidism)

Phentermine stimulates the central nervous system, which can further increase metabolism and heart rate. For people with hyperthyroidism, this added stimulation may lead to excessive thyroid hormone activity, worsening symptoms such as rapid heartbeat, nervousness, and high blood pressure.

2. People with Glaucoma

Phentermine can raise eye pressure, making it dangerous for people with glaucoma. The medication’s stimulant properties may worsen optic nerve damage, potentially worsening vision loss. Those with a history of glaucoma should consult their doctor before considering phentermine.

3. Pregnant and Breastfeeding Women

Phentermine is not recommended for use during pregnancy because it can increase the risk of fetal development issues. Plus, phentermine can pass into breast milk, potentially affecting a nursing infant. Women who are pregnant, planning to become pregnant, or breastfeeding should avoid phentermine and discuss alternative weight-management strategies with their doctor. 

4. People with a History of Heart Disease

Since phentermine can increase blood pressure and heart rate, people with conditions like coronary artery disease, arrhythmias, or a history of stroke should avoid taking it. The medication may put extra strain on the cardiovascular system, raising the risk of complications. 

5. Those with Severe Anxiety or Mental Health Conditions

Because phentermine stimulates the nervous system, it may worsen symptoms of anxiety, panic disorder, or other mental health conditions. People with a history of severe anxiety, bipolar disorder, or schizophrenia should consult a healthcare provider before using phentermine.

6. People with a History of Substance Abuse

Phentermine has stimulant-like effects similar to amphetamines, which means it carries a risk of dependence and misuse. The rate of misuse is low, but people with a history of drug or alcohol addiction should still speak to their healthcare professional before taking phentermine.

7. People Taking Certain Medications

Phentermine can interact negatively with medications such as monoamine oxidase inhibitors (MAOIs), antidepressants such as SSRIs, and other stimulant drugs. These interactions can increase the risk of dangerously high blood pressure or serotonin syndrome, a life-threatening condition. Always tell your doctor about all medications and supplements you are taking before starting phentermine.

If you fall into any of these categories, exploring alternative weight loss strategies that align with your health needs is important. A healthcare provider can help determine whether you'd benefit from a different weight loss medication.

The Bottom Line

Phentermine is FDA-approved and considered a safe and effective weight loss medication, but it comes with potential side effects, including anxiety in some people. While many symptoms (like dry mouth and headaches) are mild, severe reactions (such as irregular heartbeat and high blood pressure) require medical attention. If you live with certain medical conditions, including anxiety disorders, talk to your doctor about whether phentermine can make your condition worse.

Learn More About How to Improve Blood Sugar Health With Signos’ Expert Advice

If you're considering taking a weight loss medication like phentermine, talk to your doctor about potential interactions and side effects. Picking the right medication is important, but so is optimizing your diet and understanding what affects your blood sugar levels. ‍Continuous glucose monitors (CGMs) can help you do just that so you can take control of your health. 

Not sure if Signos is right for you? Take this quiz to find out! Interested in learning more about nutrition and healthy eating habits? Check out more articles on our blog.