Phentermine (brand names: Adipex-P, Ionamin, Lomaira) is a prescription weight loss drug, intended for short-term treatment (up to 12 weeks) in overweight or obese individuals. However, healthcare providers may prescribe phentermine to be used beyond the 12-week duration based on their clinical judgment.

Phentermine can be used alone or in combination with topiramate. The combination pill containing phentermine and topiramate is called Qsymia. Phentermine can result in weight loss when used in combination with a healthy lifestyle consisting of a calorie-deficit diet and exercise program. This weight loss drug is approved for use in adults and children 17 years and older. This medication should not be used in children under 17 years of age.

How Does Phentermine Make You Lose Weight?

Phentermine is a central nervous system stimulant and belongs to a drug class called anorectics. By stimulating the release of norepinephrine, it works to suppress appetite and increase energy expenditure. This results in reduced caloric intake and weight loss when used in combination with diet and exercise. Even if weight loss is achieved with phentermine, maintaining a healthy lifestyle is essential to sustain the weight loss. Find out Is Phentermine a Safe Weight Loss Treatment?

Learn more about “How Do Prescription Weight Loss Medications Work?”

Expected Weight Loss on Phentermine

An open-label study was conducted in Mexico, and it included more than 900 participants. This study found that about 85% of participants achieved at least 5% weight loss after six months of using phentermine. For example, the starting weight is 200 pounds (lbs), and 5% weight loss equates to 10 lbs after six months.

Common Misconceptions About Phentermine and Weight Loss

Myth: Phentermine is a quick fix for fast weight loss.

Fact: The weight loss effects of phentermine happen only when it is combined with reduced calorie intake and exercise. If you continue eating more calories or don’t exercise, it is unlikely that you will get the benefits of this drug.

Myth: Anyone can take phentermine to lose weight.

Fact: Doctors prescribe phentermine to individuals 17 or older who have a body mass index (BMI) of 30 kg/m2 or higher or a BMI of 27 kg/m2 or higher and one or more obesity-related health problems such as high blood pressure, high cholesterol, or diabetes.

Myth: You can take phentermine indefinitely to continue losing weight.

Fact: Phentermine is generally used for a short period of time (up to 12 weeks); your doctor may prescribe phentermine for longer than 12 weeks if deemed appropriate. This medicine can cause serious side effects such as increased blood pressure, heart palpitations, chest pain, cardiovascular disease, valvular heart disease, and primary pulmonary hypertension. It also is a controlled substance and has the potential for drug abuse. Additionally, the effectiveness of phentermine may decrease after the first few weeks.

Factors That Affect Weight Loss on Phentermine

There can be various reasons for not losing weight on phentermine.

Age, Sex, and Current Weight

If you are discouraged about not losing weight on phentermine, make sure you’re not comparing yourself to others who have taken this weight loss drug. How quickly you lose weight on phentermine depends on several factors, such as your age, sex, and starting weight.

Generally speaking, younger people lose weight more quickly than older people due to a faster metabolism. Males lose more weight than females of a similar height and weight because of a higher muscle mass, resulting in greater energy expenditure. People who are extremely overweight tend to lose weight more quickly than those who are only slightly overweight.

Lifestyle and Diet

One of the most common reasons people don’t lose weight on phentermine is that they fail to make dietary changes and/or include daily exercise in their routine. Phentermine only works when energy expenditure exceeds caloric intake.

The best diet to eat while on phentermine is one that excludes or limits unhealthy fats (saturated and trans fats), processed snacks, sugary foods, and sugary drinks. Instead, it is recommended to eat more fresh fruits and vegetables, whole grains, lean proteins, and healthy fats. Combining these dietary changes with an exercise program that includes cardio and strength training will provide the best chance of success in losing weight while on phentermine.

Read more: How Many Steps Should I Walk a Day?

Dosage

The usual adult dose of phentermine is one 37.5-mg tablet or capsule daily taken before breakfast or 1-2 hours after breakfast. Your doctor may prescribe phentermine 15 mg or 30 mg to take approximately 2 hours after breakfast. Taking the medicine later in the day can result in insomnia (difficulty falling and staying asleep).

Make sure you understand the dosage instructions given by your doctor. You may not be losing weight on phentermine because you are taking a low dose (lower than prescribed) or you are missing or skipping doses.

Side Effects

Some people skip doses due to side effects, which can make phentermine less effective. Common, less serious side effects of phentermine include dry mouth, a bad taste in the mouth, vomiting, diarrhea, and constipation. Tell your doctor if these side effects become severe or do not go away after a few days. Your doctor may adjust your dose or switch you to a different medication.

Significant side effects include increased blood pressure, tachycardia, and other heart problems. Be sure to discuss your medical history with your doctor prior to starting phentermine.

Medication Interactions

Combining phentermine with other medications can affect how well phentermine works or may cause additive side effects. Before starting phentermine treatment, give your doctor a complete list of all your other medicines, including prescription drugs, over-the-counter medicines, dietary supplements, and herbal remedies. This will prevent potential drug interactions that can make phentermine less effective or cause unnecessary side effects.

How to Boost Weight Loss with Phentermine

Set Realistic Expectations

The weight loss effects of phentermine can vary from person to person. So, it's important not to get discouraged and stop taking the medicine if you don’t get results right away. Remember, phentermine is not a magic bullet. Losing weight on phentermine takes work, including diet and exercise, to get the best results.

Talk to your health care provider about how much weight you can expect to lose on phentermine. Make a note of your starting weight and waist circumference and set a weight loss goal. It may be helpful to create a SMART goal (specific, measurable, achievable, relevant, and time-bound). Your first goal may even be to avoid more weight gain. It is important to routinely reflect on your progress, both your successes and setbacks, to ensure you can reach your goal.

Even if you ultimately want to lose more weight, do not get discouraged if you're only able to lose a small amount of weight initially. Studies suggest that losing just 5-10% of your body weight and staying at that weight is an extremely good result with important health benefits.

Adjust Your Eating Habits

As mentioned, eating fewer calories is the key to losing weight on phentermine. There are several ways to help you achieve this and make healthier choices:

Identify triggers (time of day, place, activity) that make you want to eat more and avoid them.
Set goals and reward yourself for achieving them. Do not use food as a reward. Rather, rewards can include buying yourself a new outfit or booking a salon session after you reach a goal. This will help reinforce and encourage healthier habits.
Reduce the amount of time you spend sedentary or inactive during the day, such as sitting at a desk.
Eat your meals in smaller plates.
Make a conscious effort to savor your food and eat more slowly.
Keep healthy snacks on hand (such as fresh fruits and nuts) for hunger pangs between meals.
Reduce portion sizes, especially for foods that are calorie-dense.
Find people who understand and support your weight loss efforts. This can be family members and friends or commercial programs.
Learn to be firm with saying no, for example, when offered unhealthy foods and beverages at parties.
Manage stress as it can trigger stress eating in some people. Also, practice positive thinking if you suffer a setback.

Get Regular Exercise

Exercise and movement play a key role alongside diet in helping you lose weight on phentermine. Regular physical activity also has other health benefits, such as reducing stress. Strategies for improving your exercise routine may include the following:

Pick an exercise plan that fits your lifestyle. You’re more likely to stick with an activity or workout you enjoy. Remember, not everyone is cut out for intense workouts in the gym. Many people enjoy group exercise activities.
Mix up your workouts to keep them interesting. Be sure to include a balanced mix of cardio, resistance training, and group fitness classes in your routine.
Keep a set of workout clothes and sports shoes in your car so that you’re prepared for any last-minute changes in plans.
Use a fitness app to log your workouts. This will help you stay on track.
Make small changes in your daily routine, like taking the stairs instead of the elevator or getting off two stops early from public transport and walking the rest of the way.
Increase accountability by telling your friends and family about your goals.

Questions and Confusions About Phentermine Not Working

What to Do When Phentermine Isn't Working?

If you feel like phentermine is not working for you, talk to your healthcare provider. They may change your dose or dosing schedule or switch you over to another weight-loss drug. Tell your doctor immediately if you develop high blood pressure, palpitations, chest pain, shortness of breath, or swelling in the legs while on phentermine.

Can You Eat Too Little on Phentermine?

It’s not a good idea to eat too little while on phentermine, as it may result in malnutrition. Be sure you are getting the appropriate nutrients through a well-balanced diet. This drug is an appetite suppressant. Therefore, your goal should be to make healthier food choices and use the effects of phentermine to avoid eating empty calories.

Weight Loss Drugs: Alternative Options to Phentermine

GLP-1 Agonists (Wegovy, Ozempic, Zepbound)

GLP-1 agonists are a group of diabetes medications that have also been approved to treat obesity. If you are not losing weight on phentermine, talk to your doctor about taking GLP-1 agonists such as liraglutide (Saxenda) or semaglutide (Wegovy), which are taken as daily or weekly shots under the skin. The FDA has also approved a dual GIP and GLP-1 receptor agonist, tirzepatide (Zepbound), for chronic weight management.

Learn more about Wegovy vs Phentermine for Weight Loss: Which is Better?

Other FDA Approved Weight Loss Medications

You can also talk to your doctor about taking one of the following FDA-approved drugs for weight management:

Orlistat (Xenical) (available over-the-counter as Alli)
Phentermine-topiramate (Qsymia)
Bupropion-naltrexone (Contrave)

These prescription weight-loss drugs work by making you feel less hungry, making you feel fuller faster, and/or affecting fat absorption and fat storage in your body.