What You Should Know About Adipex-P Side Effects

Adipex-P (Phentermine) is one of the most commonly prescribed weight loss medications in the United States. Like all medications, it comes with potential side effects — some mild and temporary, others more serious. Understanding what to expect can help you use Adipex-P safely and know when something needs medical attention.

What Is Adipex-P?

Adipex-P is the brand name for Phentermine Hydrochloride, a prescription weight loss medication classified as a sympathomimetic amine. It works by stimulating the release of norepinephrine in your brain, which suppresses your appetite and helps you eat less. It's FDA-approved for short-term use (a few weeks) in adults with a BMI of 30+ or 27+ with weight-related health conditions.

For a full overview, see our guide on what Adipex-P is and how it's used.

Common Side Effects

Most people who take Adipex-P experience at least one or two mild side effects, especially during the first week or two. These are usually manageable and tend to improve as your body adjusts:

• Dry mouth — One of the most frequently reported side effects. Drinking extra water and chewing sugar-free gum can help.
• Insomnia — Phentermine is a stimulant, so it can make it hard to fall asleep. Taking your dose early in the morning (before or shortly after breakfast) reduces this risk.
• Constipation — Eating more fiber, drinking plenty of water, and staying active can help keep things moving.
• Increased heart rate — A slightly elevated resting heart rate is common. Your doctor may monitor this at follow-up visits.
• Restlessness or nervousness — Some people feel jittery or on edge, similar to drinking too much coffee.
• Dizziness — Can occur especially when standing up quickly. Take your time changing positions.
• Headache — Usually mild and temporary. Over-the-counter pain relievers can help if needed.
• Unpleasant taste — Some patients report a metallic or bitter taste in their mouth.
• Diarrhea — Less common than constipation but can occur in some patients.
• Elevated blood pressure — Your doctor should check your blood pressure regularly while you're taking Adipex-P.

If any of these side effects become bothersome or don't improve after the first couple of weeks, talk to your doctor. They may adjust your dose — for example, switching from the standard 37.5 mg tablet to a lower 8 mg dose (Lomaira) taken three times daily.

Serious Side Effects — When to Call Your Doctor

While rare, some side effects of Adipex-P require immediate medical attention. Call your doctor or go to the emergency room if you experience any of the following:

• Chest pain or pressure — Could indicate a cardiovascular problem.
• Shortness of breath — Especially if it comes on suddenly or gets worse with exertion. This could be a sign of primary pulmonary hypertension, a rare but serious condition linked to appetite suppressants.
• Heart palpitations or irregular heartbeat — Phentermine can increase heart rate, but fluttering, pounding, or skipped beats need evaluation.
• Severe headache or vision changes — May signal dangerously high blood pressure.
• Swelling in your legs or ankles — Could indicate heart or valve problems.
• Confusion, agitation, or psychosis — Rarely, Phentermine can cause psychiatric symptoms, especially at higher doses or in people with a history of mental health conditions.
• Seizures — Extremely rare but requires emergency care.
• Signs of allergic reaction — Hives, difficulty breathing, or swelling of the face, lips, tongue, or throat.

The FDA label for Adipex-P specifically warns about the risk of primary pulmonary hypertension and valvular heart disease. While these are very rare with short-term use, they're the reason Phentermine is only approved for a few weeks at a time.

Side Effects in Specific Populations

Pregnant or Breastfeeding Women

Adipex-P is contraindicated in pregnancy (FDA Category X) and during breastfeeding. Weight loss during pregnancy offers no benefit and can cause harm to the fetus. If you become pregnant while taking Adipex-P, stop the medication and contact your doctor immediately.

Older Adults

Elderly patients may be more sensitive to Phentermine's effects on blood pressure and heart rate. Doctors should use caution and may start with a lower dose.

Patients Under 16

Adipex-P is not recommended for children under 16 years of age. Safety and effectiveness have not been established in this age group.

People with Heart Conditions

Adipex-P is contraindicated in patients with a history of cardiovascular disease, including coronary artery disease, stroke, arrhythmias, congestive heart failure, and uncontrolled hypertension. If you have any heart condition, make sure your doctor knows before starting this medication.

How to Manage Side Effects

Here are some practical tips for handling the most common side effects:

• For dry mouth: Drink water throughout the day, chew sugar-free gum, and avoid caffeine and alcohol which can make it worse.
• For insomnia: Take Adipex-P first thing in the morning — no later than 10 AM. Avoid caffeine in the afternoon and evening. Establish a regular bedtime routine.
• For constipation: Increase your fiber intake (fruits, vegetables, whole grains), drink at least 8 glasses of water daily, and stay physically active.
• For jitteriness: Cut back on caffeine. If you're taking the 37.5 mg dose, ask your doctor about switching to a lower dose.
• For elevated blood pressure: Attend all follow-up appointments so your doctor can monitor your blood pressure. Reduce sodium intake and stay active.

Don't stop taking Adipex-P abruptly without talking to your doctor first. While Phentermine has lower abuse potential than amphetamines, stopping suddenly after regular use can sometimes cause fatigue and mood changes.

What About Long-Term Side Effects?

Adipex-P is FDA-approved for short-term use only — typically a few weeks. The longer you take it, the higher the potential risk of side effects like dependence, tolerance (where the drug stops working as well), and cardiovascular problems. Your doctor should reassess your treatment regularly and may discontinue the medication if you're not seeing results or if side effects become concerning.

If you need longer-term weight management medication, ask your doctor about options like Qsymia, Contrave, or Wegovy, which are approved for extended use.

Final Thoughts

Most Adipex-P side effects are mild, temporary, and manageable with simple strategies. The most important thing is to stay in touch with your doctor, attend follow-up appointments, and report any unusual symptoms right away. Knowing what to watch for puts you in control of your treatment.

If you're currently looking for Adipex-P, Medfinder can help you find a pharmacy near you that has it in stock.