Phentermine for Weight Loss: Benefits, Side Effects, Dosage, & More

What Is Phentermine?

Phentermine is a stimulant weight loss medication that acts as an appetite suppressant. It is approved by the FDA for short-term use (up to 12 weeks consecutively) for individuals ages 17 years and older. Phentermine is used for weight loss in combination with diet and exercise modifications, and it is FDA-approved for use in patients who have a body mass index (BMI) over 30 or over 27 with other cardiovascular disease risk factors. Phentermine is a generic medication but it is also available as brand names Adipex-P and Lomaira.¹

How Does Phentermine Work for Weight Loss?

Phentermine is thought to work by affecting the levels of a neurotransmitter called norepinephrine, a body chemical linked with the “fight or flight” response. Through its stimulant effects, phentermine promotes a feeling of fullness which may reduce food intake. By acting on the appetite pathway in the brain, phentermine sends signals to the gut that decrease hunger. Phentermine itself does not increase metabolism or increase fat burning.

Side Effects of Phentermine

Because of its stimulant effects, phentermine can cause a number of side effects. Hypertension (high blood pressure), tachycardia (rapid heart rate), and palpitations are common. These are thought to occur because of increased levels of norepinephrine. Anxiety and nervousness are common, as well as insomnia and headache.

With increased blood flow to the brain and the cardiovascular system, less blood flow is directed to the gastrointestinal tract. This not only promotes fullness, but it also can cause nausea, dry mouth, and constipation.

Many patients notice that phentermine’s side effects, particularly appetite suppression, decrease over time. This drives the need for a gradual dose increase, as described below. In many cases, the tachycardia and palpitations are temporary, as well as long as patients remain on the same dosage.

On the other hand, many of the side effects (including hypertension, dry mouth, and constipation) are dose-dependent, meaning they may increase in severity as the medication dosage increases. Most of the adverse effects patients experience are perceived as mild to moderate and do not lead to discontinuation of the medication.

Common Side Effects of Phentermine

Because it acts as a stimulant, cardiovascular side effects with phentermine are common, such as tachycardia, palpitations, and elevated blood pressure. Restlessness, anxiety, and tremor also occur frequently. Phentermine’s side effects usually worsen as the dosage increases. The adverse effects are often severe enough to limit the dosage or prompt discontinuation of the drug entirely.

Common side effects of phentermine include:*²

• Palpitations and tachycardia
• Elevated blood pressure
• Restlessness
• Headache
• Tremor
• Insomnia
• Dry mouth
• Constipation

*All medications, including phentermine, can cause side effects which can be mild or serious. This is not a comprehensive list of all possible side effects. You should talk with your doctor about any questions you have and to understand the potential side effects and benefits of any medication.

Serious Side Effects of Phentermine

Of the known phentermine side effects, some of them may be serious and require immediate medical attention. Severely elevated blood pressure, especially in the presence of chest pain, should be evaluated immediately. Shortness of breath or swelling of the hands and feet while taking phentermine may indicate a problem with your heart. Signs of yellowing of your skin or eyes, called jaundice, may be a sign of liver damage as a result of taking phentermine. If any skin rash, itching, hives, or difficulty swallowing occurs while on phentermine, seek medical attention right away.

Serious side effects of phentermine include:*¹

• Severe hypertension
• Coronary ischemia
• Pulmonary hypertension
• Chest pain
• Allergic reaction
• Hepatotoxicity
• Impaired reaction time and cognition
• Withdrawal effects following long-standing use

Phentermine can harm an unborn fetus. You should not take phentermine if you are pregnant or trying to become pregnant. If you become pregnant while you are taking phentermine, discontinue the medication immediately and notify your physician.

*All medications, including phentermine, can cause side effects which can be mild or serious. This is not a comprehensive list of all possible side effects. You should talk with your doctor about any questions you have and to understand the potential side effects and benefits of any medication.

Phentermine for Weight Loss Warnings & Precautions

Phentermine should be used with caution in some patient populations, especially those with preexisting cardiovascular conditions, like heart disease or uncontrolled hypertension. Other cardiac conditions, sometimes fatal, can occur, including primary pulmonary hypertension and valvular heart disease. Caution should also be given to phentermine’s risks of tolerance, addiction, and abuse.

Warnings and precautions for taking phentermine include:¹

• Risk of addiction: Phentermine is a stimulant drug with a biochemical structure similar to amphetamine, so it carries the risks of tolerance and addiction. Weight loss drugs are also highly desirable, increasing their appeal and potentially increasing the risk of abuse.
• Allergic reactions: Allergic reactions to any of phentermine’s ingredients can occur, including urticaria, noted as a rash on the skin. Mild cases will generally resolve with discontinuation of the drug.
• Use with hypertension: Patients with uncontrolled hypertension should not take phentermine. Blood pressure should be regularly monitored during the course of treatment.
• Valvular heart disease: Phentermine may contribute to valvular heart disease, either as monotherapy or in combination with other stimulant medications.
• Primary pulmonary hypertension: Phentermine has been associated with primary pulmonary hypertension, a rare but often fatal condition that may present with shortness of breath, chest pain, passing out, or leg swelling. Seek care immediately if these symptoms occur while taking phentermine.
• Use with other weight loss medications or supplements: Do not use phentermine with other over-the-counter or prescribed weight loss drugs without checking with your doctor or pharmacist, as the safety and efficacy have not been determined.
• Effect on the ability to do potentially hazardous tasks: Use with caution while driving or operating heavy machinery, as phentermine can impair coordination and reaction time.
• Usage with alcohol: Phentermine should not be taken while consuming alcohol. Their combination increases the risk of adverse cardiovascular side effects.
• Use in patients on insulin or oral hypoglycemic drugs for diabetes: Taking phentermine with other glucose-lowering medications may increase their hypoglycemic effect. You may require a lower dose of diabetes medications if taken with phentermine. Ask your doctor before taking these types of medications together.

Phentermine Dosages for Weight Loss

Phentermine is FDA-approved for short-term use (up to 12 weeks) for weight loss in patients 17 years of age and older, who have a (BMI) over 30 or over 27 with other cardiovascular disease risk factors. Mild-to-moderate kidney disease does not affect the dosage, but more severe kidney disease requires dosage reductions.¹

The starting dosage for patients 17 years and older is typically 15 to 37.5 mg once daily, taken each morning before breakfast or 1-2 hours after breakfast. It is best practice to prescribe the lowest effective dose of phentermine, as well as to consider other risk factors. The optimal dosage may vary depending on other health conditions, tolerance, weight loss achieved, and more.¹

Phentermine is available as tablets that you swallow in strengths of 8 mg, 15 mg, 30 mg, and 37.5 mg. It is also available as an orally disintegrating tablet (ODT) in strengths of as well as capsules that are 15 mg, 30 mg, or 37.5 mg.

How Do I Take Phentermine for Weight Loss?

Phentermine dosages are determined by a healthcare professional and will be tailored to the individual to provide a response to achieve weight loss with the lowest effective dose. It’s important to take the dose of phentermine prescribed by your healthcare professional. You should take your phentermine dose in the morning with or without food.

What Happens If I Miss a Dose of Phentermine?

If you miss a dose of phentermine, take it as soon as you remember, unless it is too late in the day or almost time for your next dose. Do not take two doses together to catch up on the missed dose. Taking more than the prescribed dosage may cause serious adverse effects. Do not stop taking phentermine abruptly unless you are instructed to do so by your provider.

If you have missed a dose and aren’t sure if you should take it, contact your pharmacist or healthcare professional.

How to Get Phentermine for Weight Loss

Phentermine is a controlled prescription medication that can only be prescribed by authorized providers who also have Drug Enforcement Agency licensure. If you believe phentermine may be right for you, discuss it with your medical provider. They will discuss with you the risks, benefits, and potential alternatives to phentermine, if applicable. If you do not meet the criteria for phentermine usage, it may not be prescribed to you.

Phentermine Vs Other Weight Loss Methods

Phentermine is only one of many medications used for weight loss. Diethylpropion is also a member of the sympathomimetic class, as is phentermine. Several combination medications are available, which also act on the appetite center in the brain, including phentermine/topiramate (Qsymia). Alternatively, Orlistat blocks fat absorption in the stomach.

Several newer classes of drugs include glucagon-like peptide-1 (GLP-1) receptor agonists and dual gastric-inhibitory peptide (GIP)/GLP-1 receptor agonists. These optimize the levels of neuroendocrine hormones in the gut. GLP-1 receptor agonists include semaglutide (Wegovy)

For some patients, metabolic and bariatric surgery may be an alternative to obesity pharmacologic treatment.

Phentermine in Combination With Lifestyle Changes

Phentermine is indicated for use in combination with lifestyle interventions, including dietary modifications and physical exercise. Phentermine has been shown to yield body weight loss of 5 to 10% over 12 to 24 weeks. However, many people regain their weight loss after stopping the medication.²

However, when phentermine is used in combination with lifestyle changes, more people maintain the 5 to 10% of body weight they lost. In patients with a history of abnormal blood sugar, levels may need to be monitored to evaluate for hypoglycemia if dietary carbohydrates are restricted while taking phentermine.²

FAQs About Phentermine for Weight Loss

There are many common questions surrounding the use of phentermine for weight loss. If you have questions about taking phentermine, be sure and ask your provider before or while you are taking it.

How Quickly Can You Lose Weight on Phentermine?

Some patients lose about 2 pounds per week on phentermine, although this can vary between patients. Most patients taking phentermine are seen by their provider at least once per month, and weight is recorded. However, weight loss on phentermine is usually reported after 12 weeks of treatment.¹

How Much Weight Can I Lose on Phentermine?

After 12 weeks of taking phentermine, the majority of patients lose at least 5% of their body weight, and about 25% lose at least 10% of their body weight.²

How Effective Is Phentermine for Weight Loss?

Individuals may lose an average of 5 to 10% of their body weight while taking phentermine. If you make successful dietary modifications and are dedicated to adequate physical exercise, you are significantly more likely to keep the weight off long-term.²

Can I Drink Alcohol While Taking Phentermine?

You should not drink alcohol while taking phentermine. Phentermine can amplify the effects of alcohol and decrease alcohol tolerance. Moreover, drinking alcohol while taking phentermine can make it harder to lose weight.

Questions to Ask Your Healthcare Provider About Phentermine for Weight Loss

If you have tried dietary modifications and physical exercise for weight loss without success, you may want to consider talking to your healthcare provider about weight loss medications, such as phentermine.

Questions to ask your care team about phentermine for weight loss include:

• How safe is phentermine for weight loss?
• How much weight can I lose while taking phentermine?
• Will phentermine allow me to lose weight and keep it off long-term?
• Can I take phentermine long-term?
• How long will it take for me to adjust to taking phentermine for weight loss?
• Am I at risk for more serious symptoms if I am already taking other medications?
• Will taking phentermine impact my other health conditions?
• Can I become addicted to phentermine?
• Can I drink caffeine while taking phentermine?
• Can I drink alcohol while taking phentermine?
• Could Adderall be compared to phentermine as an alternative weight loss treatment?

How to Know if Phentermine for Weight Loss Is Right for You

If you are 17 years of age and older and you have a BMI over 30 or over 27 with other cardiovascular disease risk factors, phentermine may be right for you. It should only be used in combination with diet and lifestyle changes to encourage weight loss, and it is only recommended for short-term use (less than 12 weeks).

Outside of approved phentermine/topiramate combinations, phentermine should not be used along with other medications for weight loss. It should also not be taken with certain psychiatric medications that increase serotonin levels.

Phentermine is not recommended for patients with certain health conditions, such as valvular heart disease, primary pulmonary hypertension (a rare lung disorder), and for those who are pregnant or breastfeeding. Caution should be used for patients with heart disease, severe uncontrolled high blood pressure, usage of alcohol, and other agents that lower blood sugar levels.