Phentermine is an amphetamine-like prescription appetite suppressant used for weight loss.
First approved in 1959, phentermine is the most-prescribed weight loss medication in the United States to treat together with diet and exercise obesity (
Mechanism of Action
Phentermine activates the central nervous system and stimulates the release of certain neurotransmitters like dopamine, epinephrine (adrenaline), and norepinephrine (noradrenaline). As a result, the central nervous system induces a “fight or flight”-like reaction, which causes increased heart rate, sweating, lack of appetite, and extra energy (
In the United States and Latin America, most phentermine-based weight loss pills contain phentermine hydrochloride as their main ingredient.
However, phentermine resin-based pills are still available in other parts of the world (such Ionamin and Duromine).
Who Takes It
This weight loss medication is an option for patients with a body mass index (BMI) greater than or equal to 30 kg/m2, which puts them in the “obese” range. Phentermine is also available to individuals with a BMI of 27-29.9 kg/m2 plus a weight-related condition like type 2 diabetes, high cholesterol, or controlled hypertension (
However, this medication is not appropriate for all overweight and obese patients. There are many contraindications for phentermine use, including, but not limited to: history of cardiovascular disease, history of addiction, history of agitated states, or current/recent use of certain medications or supplements (
This recommendation is based on the original drug trials in which participants were monitored over 12 weeks, and the safety of more prolonged medication use was not assessed.
However, some doctors will prescribe phentermine for more extended periods, or prescribe it several times within a specified period with month-long breaks between each period. Such a treatment plan must be evaluated on a case-by-case basis.
Phentermine weight loss pills are available in both tablet and capsule form, in doses ranging from 8 – 37.5 milligrams.
The lowest dose tablets (Lomaira 8mg) can be taken up to three times per day, while higher-dose tablets and capsules can only be taken 1-2 times per day. Exact phentermine dosage and schedule varies by individual.
|8 mg tablet||15-18.75 mg capsule||30-37.5 mg|
capsule or tablet
The maximum dose of phentermine hydrochloride is 37.5 mg per day.
Phentermine is a highly-effective weight loss medication, but it also has a long list of potential side effects.
- Chest pain
- Irregular (fluttering) or pounding heart rate
- Shortness of breath
- Lightheadedness or fainting
- Swelling of the ankles, legs or feet
- Stroke-like symptoms, such as slurred speech or one-sided weakness
- Extreme happiness or sadness
Most patients report fewer side effects as the treatment progresses. Lower dose pills or alternate schedules can also help alleviate the bothersome reactions faster.
Despite the growing necessity for pharmaceutical weight loss options, many doctors and potential patients remain skeptical of phentermine due to its troubled past.
During the 1980-90s, doctors commonly prescribed an off-label combination of fenfluramine (fen) and phentermine (phen) for weight loss (
Phentermine has remained available with a warning on the package insert (
Frequently Asked Questions
Is Phentermine an Amphetamine?
No, phentermine is not an amphetamine.
However, it does have a chemical structure similar to amphetamines, so phentermine weight loss pills can cause a false positive on 5-panel drug screens.
Is Phentermine Safe?
Phentermine is an FDA-approved medication, and is generally safe when taken as prescribed.
Phentermine, like all other medications, can cause side effects. However, when doctors prescribe a weight loss drug such as phentermine, it is because the benefits of losing weight to reach a healthy BMI outweigh the possible side effects.
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