Who can take phentermine for weight loss?

In this article:

• What is phentermine?
• How does phentermine work for weight loss?
• How long should you take phentermine?
• Phentermine dosage
• Is phentermine safe?

In this article

• What is phentermine?
• How does phentermine work for weight loss?
• How long should you take phentermine?
• Phentermine dosage
• Is phentermine safe?

Is it safe to take phentermine for weight loss?

Phentermine is a prescription diet pill that suppresses appetite. People who are obese with a body mass index (BMI) greater than 30 kg/m2 or people who have weight-related complications with a BMI of 27 to 29.9 kg/m2 could benefit from drug treatment like phentermine. Phentermine is effective, generally safe to take short-term (three months) for most people, and inexpensive. It’s important to understand diet pills like phentermine aren’t advised for reasons other than chronic weight management issues.

What is phentermine?

Phentermine is the oldest prescription weight-loss pill used today to treat obesity. It’s also the drug that’s most commonly prescribed for this purpose, even though there are newer options on the market.

First, it’s popular because it’s effective. Studies have found it can bring about a loss of 5% to 10% of body weight over 12 weeks when taken as part of a treatment plan that includes a low-calorie diet and increased physical activity.

Second, it’s inexpensive, according to Caroline M. Apovian, MD, a professor at Boston University School of Medicine who specializes in endocrinology, diabetes, nutrition, and weight management. Many insurance companies won’t pay for weight loss medication or treatment, despite plenty of research confirming obesity’s health dangers. Phentermine may be the only viable option for people with limited income.

Phentermine is available as a generic drug and under the brand names Adipex-P and Lomaira.

Related: What is phentermine? | Where can I get phentermine?

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How does phentermine work for weight loss?

In medical lingo, phentermine is an anorectic, which simply means it curbs hunger. It does this, in part, by triggering the release of certain brain chemicals in the area of the hypothalamus that controls appetite, explains Karl Nadolsky, MD, a board-certified endocrinologist who specializes in diabetes, metabolism, and obesity at Spectrum Health in Grand Rapids, Michigan, and a Fellow of the American College of Endocrinologists (FACE).

Hence, phentermine helps decrease appetite by affecting the central nervous system.

How long should you take phentermine? 

Phentermine is only approved for short-term use. The long-term safety and effectiveness of phentermine has never been studied—nor will it be, according to Dr. Apovian, who explains that the type of rigorous research necessary would be far too expensive.

People taking phentermine typically experience weight loss soon after starting it. Clinical trials have determined that weight loss is greatest in the first weeks of treatment. Each individual experiences weight loss from their baseline weight, which can vary from person to person. One study found that over the course of 14 weeks of phentermine treatment, individuals averaged just over 1 pound of weight loss per week.

Phentermine dosage

Determining the best phentermine dose for you will depend on many factors. Taking phentermine alone without diet and lifestyle modifications will not likely lead to significant weight loss at any dose. It is difficult to correlate a specific phentermine dose to a specific amount of weight loss because there are so many other factors that are key to successful weight loss, even when taking an anti-obesity drug. A reduced-calorie diet and increased physical activity are two very important factors to weight loss. When dieting, every person will do these to a different extent, making it difficult to relate a dose of the drug to an amount of body weight loss. 

One of the most recent clinical studies performed to evaluate phentermine-related weight loss was in Korea in 2005. This study involved 119 obese patients with comorbidities of high blood pressure, diabetes, and high cholesterol, and the study followed them for 12 weeks. At the 12-week mark, almost 74% of patients had lost at least 5% of their initial body weight. This weight reduction was not associated with improvements in blood sugar or cholesterol readings. Blood pressure changes were similar when the phentermine group was compared to those who had received placebo, however, those who received phentermine had significantly elevated pulse readings.

Phentermine is available in oral capsule dosage forms of 15mg, 30mg, and 37.5mg. There are also phentermine tablets available as 8mg and 37.5mg (equivalent to 30mg of phentermine base). Typical dosing for patients is 15mg to 37.5mg daily and may be taken in divided doses (once, twice, or three times per day), though your healthcare professional will determine the best dose for you.

Adverse events like insomnia, dry mouth, and dizziness may increase with higher doses of phentermine. 

Is phentermine safe?

Phentermine has been considered a controversial appetite suppressant. The diet pill has a risky reputation for two main reasons:

1. Phentermine is an amphetamine-like medication. 

It’s chemically similar to amphetamines. So, the United States Food and Drug Administration (FDA) classifies it as a Schedule IV drug (i.e., a federally controlled substance), because of concerns for potential abuse or addiction. Dr. Nadolsky points out tha phentermine is not an amphetamine. “Phentermine appears to be safe—even for long-term use,” he says. “And in my practice, I’ve never seen or heard of a problem with phentermine.”

2. Phentermine was half of the drug duo fen-phen.

In the 1990s, physicians prescribed phentermine with fenfluramine or dexfenfluramine, a pairing nicknamed fen-phen. Fen-phen became a miracle weight-loss craze until the FDA realized fenfluramine and dexfenfluramine were causing serious heart problems and pulled them from the market in 1997. Phentermine was not determined to be linked to the more serious side effects and cardiac events and, therefore, it was allowed to remain on the market as a short-term monotherapy for obesity.

Your healthcare provider will consider your cardiac risk when deciding if phentermine is the right medication for you. If you have a medical history of irregular heartbeat, pulmonary hypertension, uncontrolled blood pressure, or the presence of other cardiovascular diseases, you may not be a candidate for phentermine treatment. If you are taking phentermine and develop any of these conditions, you should stop the drug and seek medical help immediately. Your doctor may consider the presence of other risk factors as well before prescribing phentermine.

Phentermine side effects

Serious adverse effects are rare. The most common side effects of phentermine include:

• Rapid heart rate
• Tingling in the hands or feet
• Dry mouth
• Sleeplessness
• Nervousness
• Constipation

You should not take phentermine if you have heart disease, including valvular heart disease, heart failure or high blood pressure, seizure disorders, severe renal disease, an overactive thyroid, or glaucoma, or are pregnant, trying to become pregnant, or breastfeeding.

Phentermine alternatives for weight loss

There are alternatives to phentermine to aid in weight loss. In fact, while phentermine is only approved for short-term use, typically 12 weeks or less, many of the most recently FDA-approved medications for weight loss are indicated for long-term use.

Alternative oral prescription medications include:

• Contrave
• Qsymia
• Xenical

Xenical, Qsymia, and Contrave are oral prescription medications indicated for long-term use for weight loss. Qsymia is actually a combination of phentermine and another drug, topiramate. Topiramate was initially an FDA-approved drug under the brand name Topamax and is indicated for the treatment of seizure disorders. Post-marketing observance correlated topiramate to weight loss which is likely due to an appetite suppression effect. While phentermine alone is only indicated for short-term use, Qsymia has been approved for long-term use to aid in weight loss and treat obesity. Qsymia and phentermine are controlled substances as categorized by the FDA. This means these prescription drugs have some potential for misuse or abuse, and patients with a history of drug abuse should not take these drugs.

Alternative injectable prescription medications include:

• Saxenda
• Wegovy

Saxenda and Wegovy are each glucagon-like peptide-1 (GLP-1) weight loss drugs. This class of drugs initially came to market to treat type 2 diabetes mellitus. Research also showed significant benefits in managing weight loss, and these two forms of GLP-1 drugs are specifically dosed and approved for long-term weight loss treatment. Other GLP-1 drugs approved to treat diabetes are sometimes used “off-label” for obesity. Off-label indicates the use of a drug for a purpose the FDA has not approved an indication for. 

RELATED: Wegovy vs. phentermine

There are over-the-counter “diet pills” and dietary supplements as well, but these have not been studied and approved by the FDA for weight loss. You should speak to a pharmacist or other health care provider before taking anything over the counter for weight loss.

The chart below provides some FDA-approved weight loss drug alternatives to phentermine. There are links to help you find the lowest price for the drug if your doctor chooses one of these options. Only your healthcare providers can decide which option is right for you.

Is phentermine right for you?

You should only take phentermine for weight loss if you are obese—with a body mass index (BMI) of over 30—or have a BMI of over 27 while having another serious medical condition caused by a chronic weight management issue like diabetes or hypertension.

The real danger of phentermine lies—not in the drug itself, but in how it may be mishandled. There are what Dr. Nadolsky refers to as “phentermine factories” that are not associated with a hospital or other bona fide medical programs. They sell phentermine directly to clients looking for fast, easy weight loss without screening for risk factors—a practice that is illegal and risky.

When prescribed as part of an overall treatment plan by a doctor who specializes in obesity or endocrinology and who will provide careful oversight, it’s unlikely phentermine will be problematic. It should always be part of a balanced program, paired with healthy lifestyle changes such as:

• a reduced-calorie, nutrient-dense diet
• increased physical activity
• behavioral changes

Without these basic foundations of healthy weight loss, you’re likely to gain back all of the pounds dropped after you stop taking phentermine.

With these keystones and appropriate medical supervision, phentermine can be highly effective as part of a short-term weight loss program.