What You Should Know About Phentermine Side Effects

Phentermine is one of the most widely prescribed weight-loss medications in the United States, and for good reason — it works. But like all medications, it comes with side effects. Most are mild and go away on their own, but some require medical attention.

This guide covers the common and serious side effects of Phentermine, who's at higher risk, and exactly when you should call your doctor.

What Is Phentermine?

Phentermine is a prescription appetite suppressant classified as a sympathomimetic amine. It's FDA-approved for short-term weight loss (up to 12 weeks) in adults with a BMI of 30 or higher, or 27 or higher with weight-related conditions. It works by stimulating the release of norepinephrine in the brain, which suppresses hunger and increases energy. For a deeper look at the medication itself, see our complete guide to Phentermine.

Common Side Effects

These side effects are reported frequently and are usually mild. They often improve within the first week or two as your body adjusts to the medication.

• Dry mouth — One of the most common complaints. Drinking plenty of water and chewing sugar-free gum can help.
• Insomnia — Phentermine is a stimulant, so it can interfere with sleep. This is why doctors recommend taking it in the morning.
• Increased heart rate — A slight increase in resting heart rate is normal. Your doctor should monitor this at follow-up visits.
• Constipation — Increasing fiber and water intake usually helps resolve this.
• Restlessness and nervousness — You may feel jittery or on edge, especially in the first few days. This is related to the stimulant effect.
• Headache — Common in the first week, usually resolves on its own.
• Dizziness — Can occur when standing up quickly. Stay hydrated and rise slowly from sitting or lying positions.
• Unpleasant taste — Some patients notice a metallic or bitter taste.
• Diarrhea — Less common than constipation, but still reported by some patients.
• Elevated blood pressure — Phentermine can raise blood pressure, so regular monitoring is important.

Serious Side Effects

These side effects are less common but require immediate medical attention. Call your doctor or go to the emergency room if you experience any of the following:

• Primary pulmonary hypertension — This is a rare but serious condition where blood pressure in the lungs becomes dangerously high. Symptoms include increasing shortness of breath, chest pain, fainting, and swelling in the ankles or legs. This can develop even after the medication is stopped.
• Heart valve problems — Rarely reported with Phentermine alone (more commonly associated with the now-banned fenfluramine combination). Symptoms include unusual shortness of breath, chest pain, and fatigue with minimal exertion.
• Severe palpitations or tachycardia — A racing heart that doesn't settle down, especially with chest tightness or pain.
• Chest pain — Any new or unexplained chest pain while taking Phentermine should be evaluated immediately.
• Severe shortness of breath — Difficulty breathing, especially during activities that didn't previously cause breathlessness.
• Swelling of legs and ankles — Can indicate fluid retention or cardiovascular problems.
• Psychosis or severe psychiatric symptoms — In rare cases, Phentermine can cause extreme agitation, hallucinations, or paranoia. Stop the medication and seek help immediately.
• Signs of dependence — Phentermine is a Schedule IV controlled substance. If you find yourself needing higher doses to get the same effect or feeling unable to stop, talk to your doctor.

Side Effects in Specific Populations

Older Adults (Over 65)

Phentermine is not recommended for patients over 65. Older adults are more likely to have cardiovascular conditions that make stimulant medications risky. If you're over 65, talk to your doctor about safer alternatives.

Pregnancy and Breastfeeding

Phentermine is Category X — contraindicated in pregnancy. It should not be used if you are pregnant, planning to become pregnant, or breastfeeding. Weight loss during pregnancy offers no benefit and may cause harm to the fetus.

Patients with High Blood Pressure

If you have mild, controlled hypertension, your doctor may still prescribe Phentermine with close monitoring. However, it's contraindicated in uncontrolled hypertension. Your blood pressure should be checked regularly while on this medication.

Patients with a History of Mental Health Conditions

Because Phentermine affects brain chemistry, patients with a history of anxiety, agitation, or psychiatric disorders should use it with caution. The stimulant effects can worsen these conditions.

How to Manage Common Side Effects

Most mild side effects can be managed with simple strategies:

• For dry mouth: Drink at least 8 glasses of water daily. Chew sugar-free gum or suck on ice chips.
• For insomnia: Take Phentermine first thing in the morning — no later than 10 AM. Avoid caffeine in the afternoon. Establish a consistent bedtime routine.
• For constipation: Eat more fiber-rich foods (fruits, vegetables, whole grains). Consider a gentle over-the-counter fiber supplement. Stay hydrated.
• For jitteriness: Cut back on caffeine. This is important — caffeine amplifies Phentermine's stimulant effects. If nervousness is severe, your doctor may lower your dose.
• For elevated blood pressure: Monitor your blood pressure at home if possible. Report any consistently high readings to your doctor.
• For headache: Stay hydrated. Over-the-counter pain relievers like acetaminophen are generally safe to use. Ask your doctor if headaches persist.

When to Call Your Doctor

Call your doctor if you experience:

• Chest pain or tightness
• Shortness of breath that's new or worsening
• Heart racing that doesn't stop
• Swelling in your legs or ankles
• Severe mood changes, agitation, or confusion
• Fainting or feeling like you might faint
• Side effects that don't improve after 1-2 weeks
• Any symptom that concerns you

Don't wait it out if something feels wrong. It's always better to call and be reassured than to ignore a warning sign.

Final Thoughts

Phentermine is an effective weight-loss medication for many people, and most side effects are mild and temporary. The key is knowing what to watch for and staying in close contact with your doctor, especially during the first few weeks.

If you have a prescription and need help finding Phentermine at a pharmacy near you, Medfinder can help you locate it in stock.