Medically Approved

What you need to know about phentermine 

Woman exercising outdoors for weight loss

This popular medication can help you shed pounds — if used the right way. 

Mark Ray

By Mark Ray

For millions of people, the weight-loss struggle is real. More than 40% of American adults live with obesity. That puts them at increased risk of heart disease, Type 2 diabetes and stroke, as well as some forms of cancer. That number is up from 30.5% just a decade ago.  

If you’re having a hard time losing extra pounds, medications such as phentermine (Adipex-p®, Lomaira™) can help. To learn more, we talked with Nupur Garg, MD, an emergency medicine doctor with Lifestyle and Family Medicine in North Haven, Connecticut. 

Why you gain weight — and how to lose it 

Everyone is different, but weight gain generally comes down to this, says Dr. Garg: “You’re overconsuming calories compared to what you’re burning. Your body is storing that in your system in the form of fat.”  

Excess fat is especially bad when it’s stored in your midsection. That type of fat is called visceral fat. “And it’s dangerous because it can cause organ damage,” she adds. That’s why you may hear that it’s better to have a pear-shaped body than an apple-shaped one.  

To lose weight, you need to burn more calories, eat fewer calories or do both. Exercise can help you burn more calories. That’s one reason you should aim for at least 150 minutes of moderate to vigorous exercise per week. Eating a healthy diet and avoiding sugar-heavy foods is the best way to lower your calorie intake. (Check out “5 ways to lose weight without counting calories” for more ideas.) 

But if diet and exercise aren’t helping you shed pounds, your doctor may suggest medication. One of the most common ones for weight loss is phentermine. Although Dr. Garg generally leans toward newer medications, she does prescribe phentermine in the right situations. 

When you go to the pharmacy, don’t forget to use your free prescription discount card. It could save you a lot of money. 

How does phentermine work? 

Phentermine is an amphetamine-like medication that can jump-start weight loss. “Phentermine works by suppressing your appetite, which can lead to lifestyle changes and eventually cause weight loss,” says Dr. Garg. But if you keep eating your usual diet, even though you don’t have much of an appetite, phentermine won’t help. 

Phentermine is sometimes prescribed along with another medication, topiramate (Topamax®, Qudexy® XR, Topamax® Sprinkle, Trokendi XR®). It’s an anti-epileptic medication that also decreases appetite and helps you feel fuller for longer after eating. There’s also a combined medication called Qsymia®

Why take 2 appetite suppressants? “They are acting on 2 different neurotransmitters,” Dr. Garg says. And that can be more effective for weight loss. 

How to use phentermine 

Phentermine should be used only for a short period of time. Most people take it for 3 to 6 weeks. Longer use isn’t recommended because it can be habit forming.  

Phentermine comes as tablets or extended-release capsules. Depending on your prescription, you may take a single dose each morning or 3 doses a day, taken 30 minutes before each meal. If you miss a pill, you should take it as soon as you remember to. But if it’s almost time for the next dose, don’t double up. It’s better to miss it. 

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Who shouldn’t use phentermine 

Your doctor can discuss whether phentermine is right for you. But according to MedlinePlus, it typically isn’t prescribed if: 

  • You have a history of heart disease, high blood pressure, arteriosclerosis, hyperthyroidism, diabetes, glaucoma or drug abuse 
  • You’re pregnant, plan to become pregnant or are breastfeeding 
  • You’re age 65 or older 

Also, talk with your doctor about any prescriptions, over-the-counter medications, supplements or herbal products you’re taking. 

Medications that shouldn’t be taken with phentermine include fluoxetine (Prozac®), paroxetine (Paxil®), sertraline (Zoloft®) and monoamine oxidase inhibitors such as isocarboxazid (Marplan®) and phenelzine (Nardil®). Taking them together can increase the effects of phentermine. That can cause side effects such as jitteriness, anxiety and racing thoughts. In rare cases, combining these medications can cause serotonin syndrome. That’s a potentially life-threatening drug interaction from having too much serotonin in your system.  

Possible side effects 

Like all medications, phentermine can cause side effects, although not everyone will experience them. Common ones include: 

  • Dizziness 
  • Feeling nervous and excitable 
  • Restlessness 
  • Headache 
  • Constipation or diarrhea 
  • Dry mouth and/or a bad taste in your mouth 
  • Insomnia 
  • Lowered interest in sex and/or erectile dysfunction 

More severe side effects can include: 

  • Tremor 
  • Shortness of breath 
  • Increased blood pressure 
  • Heart palpitations 
  • Chest pain 
  • Swelling in the legs and ankles 
  • Difficulty doing exercises you can ordinarily do 

Call your doctor if you have any severe side effects — or mild ones that don’t go away.  

When used as directed, phentermine can be very effective in helping you shed unwanted pounds. But Dr. Garg says there’s still no such thing as a magic pill. You also have to put in work and make changes to your diet and exercise routine that will support weight loss. 

Look for your prescription on the Optum Perks discount app before you head to the pharmacy. You could find medication coupons for up to 80% off. 

Additional sources :
Causes of obesity: Centers for Disease Control and Prevention 
Taking phentermine: Mayo Clinic 
How to know if phentermine isn’t right for you: MedlinePlus