Key takeaways

Phentermine is an effective, inexpensive prescription diet pill primarily used to treat obesity by suppressing appetite and is safe for short-term use.
It works by affecting the central nervous system to decrease appetite and is most effective when combined with a low-calorie diet and increased physical activity.
Phentermine has a high discontinuation rate due to side effects such as insomnia, dry mouth, and dizziness, and its long-term safety and effectiveness have not been studied.
Alternatives to phentermine for long-term weight loss include FDA-approved medications like Xenical, Qsymia, Contrave, Saxenda, and Wegovy, with options for both prescription and over-the-counter solutions.

Is it safe to take phentermine for weight loss?

Phentermine is a prescription diet pill that suppresses appetite. For people who are obese, or have a weight-related medical condition, this appetite suppressant can be a life-changing drug. Phentermine is effective, safe to take short-term (three months), and inexpensive. For people who just want to fit into their skinny jeans, phentermine would never be medically advised or prescribed.

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 under the brand names Adipex-P and Lomaira.

Related: What is phentermine?

How does phentermine work for weight loss?

Phentermine is, in medical lingo, 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.

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 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 and 30mg. There is also a tablet form which is a 37.5mg dose, but is the equivalent of 30mg of phentermine base. There are also 8mg and 18.75mg dosage forms available. Typical dosing for patients is 15mg to 30mg daily, though your healthcare professional will determine the best dose for you.

Adverse events like insomnia, dry mouth, and dizziness will increase with higher doses of phentermine. Phentermine has a high discontinuation rate due to adverse events.

Is phentermine safe?

Phentermine is an often maligned medication. 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 U.S. 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, 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 phen-fen.

In the 1990s, physicians prescribed phentermine with fenfluramine or dexfenfluramine, a pairing nicknamed phen-fen. Phen-fen became a miracle weight-loss craze, until the FDA in 1997 realized fenfluramine and dexfenfluramine were causing serious heart problems and pulled them from the market. Phentermine was not determined to be linked to the more serious side effects and cardiac events and therefore was allowed to remain on the market as 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 history of irregular heartbeat, pulmonary hypertension, uncontrolled blood pressure, or the presence of other cardiovascular disease, 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, depression, 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.

Xenical, Qsymia, and Contrave are oral prescription medications indicated for long-term use for weight loss. Qsymia is actually phentermine combined with 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, Qysymia has been approved for long-term use to aid in weight loss and treat obesity. Contrave and phentermine are controlled substances as categorized by the FDA. This means the drugs have some potential for misuse or abuse, and patients with a history of drug abuse should not take these drugs.

Saxenda and Wegovy are each glucagon-like peptide-1 (GLP-1) weight loss drugs. This class of drugs initially came to market to treat Diabetes Mellitus Type 2. Research also showed significant benefit 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 use of a drug for a purpose the FDA has not approved an indication for.

There are over-the-counter “diet pills” 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 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 health care providers can decide which option is right for you.

Phentermine alternatives
Drug	Drug coupon
Contrave (naltrexone/ bupropion	Savings options
Qsymia (phentermine/ topiramate)	Savings options
Xenical or Alli (orlistat)	Savings options
Saxenda (liraglutide)	Savings options
Wegovy (semaglutide)	Savings options
Tenuate (diethylpropion)	Savings options

Is phentermine right for you?

You should only take phentermine or weight loss if you are obese—with a body mass index (BMI) of over 30—or a BMI of over 27 if have another serious medical condition caused by obesity, 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 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.