Disclaimer: None of the information in this article constitutes medical advice, and is just the opinion of the writer(s). We recommend that patients follow their doctor’s guidance in regard to prescription medication.
Phentermine is a prescription medication approved by the Food and Drug Administration (FDA) for weight loss. It’s a stimulant, and is one of the most commonly-prescribed pharmaceutical treatments for obesity.
The drug's official name is phentermine hydrochloride, or phentermine HCL, but it's typically shortened to phentermine.
Phentermine is the name of the generic form of the drug, and the branded versions are Adipex-P and Lomaira. These terms all refer to the same active drug ingredient, so we'll use them interchangeably throughout this article.
But is phentermine proven in research studies to cause weight loss? And if so, how much? Does the drug have the potential to cause side effects? And how do real users rate and describe the effects of phentermine?
In this article we’ll answer all of these questions and more, as we review clinical studies on phentermine to determine if the drug is effective for weight loss, and if so, how much weight loss it causes.
We'll feature unsponsored patient reviews including before-and-after images, discuss side effects, and explain if alcohol can be used while taking phentermine.
Does Phentermine Cause Weight Loss?
There have been many medical studies, both short-term and long-term, on the effectiveness of phentermine for weight loss.
A clinical trial published in the Yonsei Medical Journal examined the effects of phentermine in obese patients.
The researchers documented that after 14 weeks, nearly 90% of the patients taking phentermine lost 5% of their body weight or more, and 50% of patients taking phentermine lost 10% body weight or more.
These figures were significantly higher than the placebo group, of which 17% lost 5% body weight or more, and 8.3% lost 10% body weight or more.
A 2019 medical review analyzed the efficacy of long-term phentermine use (up to 24 months of continuous use).
The study authors found that patients taking phentermine for over 12 months lost 7.4% more weight than a reference group that wasn’t taking the drug.
A 2014 meta-study examined data from six clinical trials on phentermine for weight loss.
The study authors noted that a statistically significant amount of weight loss was recorded across all trials. The average weight loss was 13.91 pounds, and the studies ranged from 2-24 weeks.
We will conclude from the available medical data that phentermine is effective for weight loss for both short-term and long-term use, which is unsurprising given that the drug is approved by the FDA for that indication.
Does Phentermine Cause Side Effects?
Phentermine does cause side effects in some patients.
Some of the most common side effects of phentermine according to medical research are: anxiety, palpitations, headache and insomnia. While these side effects aren’t life-threatening, they can be uncomfortable.
The drug's FDA label lists a warning about using phentermine and other medications.
Phentermine should not be used with other weight loss drugs, whether prescription drugs or over-the-counter (OTC) drugs, for more than "a few weeks" because the long-term safety of phentermine in combination with other weight loss drugs has not been established.
Phentermine's FDA label does not contain a "black box" warning, which is the most severe level of warning issued by the agency for side effects with life-threatening potential.
We consider this to be a good sign, given that many of the prescription weight loss medications we've reviewed on Illuminate Health (like Mounjaro) do have a black box warning.
Real People Try Phentermine
A YouTube creator named Norisbel Cayo has a review of phentermine where she claims to have lost 20 pounds in one month, and includes before-and-after images:
A YouTube creator named "KeikoBeauty" claims to have lost 10 pounds in two weeks while taking phentermine:
How Much Does Phentermine Cost?
The retail price of phentermine is around $15 at the time of updating this article, according to GoodRx.
It's important to remember that phentermine is a generic drug, and generic drugs are often much cheaper than their brand-name alternatives, but should be equally effective.
A medical review published in the PLOS Medicine journal compared the effectiveness of generic and brand-name medications, and concluded that generic drugs were just as effective.
As a point of comparison, Adipex-P currently costs around $85 according to GoodRx, even though it contains the same active ingredient as phentermine.
Phentermine is not currently available for purchase over-the-counter (OTC) in the US. It requires a prescription from a medical professional.
Health insurance may fully or partially subsidize the cost of phentermine, so we recommend that patients considering phentermine for weight loss speak with both their doctor and insurer about what the most cost-effective option is.
Patients Review Phentermine
Phentermine has been reviewed over 2,900 times on Drugs.com, which is a website that allows prescription drug users to publish reviews and ratings of medications they're taking.
We cannot verify the accuracy or authenticity of any reviews on this site.
The average rating for phentermine is currently 8.7/10.
The top positive review is from a user named "Michelle RR" who claims that the drug caused significant weight loss, and that the weight was not regained even after discontinuing the medication:
"I took [phentermine] for 1 year and lost 90lbs. I changed my eating habits and starting doing light exercise 3 days a week. I've been off [phentermine] for 3 months and haven't gained any weight back."
The top negative review is from a user named "Michelle" who claims that the drug was effective for weight loss but caused uncomfortable side effects:
"Yes it works for weight loss but it’s not worth the long term side effects . You will end up addicted to this medication, it is a stimulant that increases your dopamine and then depletes it once you stop which makes you severely depressed when you quit . If you have anxiety or might be bipolar don’t take this medication as it can make you very manic , euphoric , and give you a false sense of invincibility".
Can You Drink Alcohol on Phentermine?
We would recommend against drinking alcohol while taking phentermine due to potential drug interactions.
Phentermine's FDA label states that "concomitant alcohol use may result in an adverse drug reaction."
The label also lists "usage with alcohol" in a "Warnings and Precautions" section.
We recommend that patients be honest with their doctor about any alcohol use issues, because these drug label warnings makes it seem like even moderate alcohol use may be a risk factor, and that alcohol use should be stopped entirely while taking phentermine.
Patients unable to quit alcohol may benefit from speaking with their doctor about alternative weight loss medications that don't have a negative interaction with alcohol.
Our Clean Weight Loss Picks
Ceylon cinnamon was described as "an effective anti-obesity agent" in a 2022 meta-analysis. The study authors concluded that effects were greater at doses at or over 3 grams daily.
Illuminate Labs Ceylon Cinnamon Extract is our standardized Ceylon cinnamon supplement which is third-party tested to ensure purity and potency.
Dietary fiber was shown in a medical review published in The Journal of Nutrition to cause 16 pounds of weight loss in 6 months when combined with moderate caloric restriction (750 calories per day below baseline).
MBG Organic Fiber Potency+ is our top fiber pick because it's certified organic, provides 7 g of fiber per serving and costs under $1.85 per serving at the time of updating this article.
Both of the products mentioned in this section are entirely free of additive ingredients that we consider to be unhealthy.
We're not suggesting these products should be used to treat any medical condition, or that they're as effective as any FDA-approved medication; rather, we're just sharing information that individuals averse to prescription medication can speak with their doctor about.
How Does Phentermine Work?
Phentermine primarily works by suppressing appetite.
When patients take phentermine pills, levels of a neurotransmitter called norepinephrine increase significantly in their brain. This has indirect effects on hunger.
A medical review suggests that phentermine's mechanism of action is to inhibit expression of a neuropeptide called neuropeptide Y which sends hunger signals to the body, but the drug's biological effects aren't fully understood yet.
What we do know is that phentermine has a similar molecular structure to amphetamine, and has similar effects such as central nervous system stimulation and increased heart rate and blood pressure.
As referenced in the above-linked medical review, phentermine causes a "continuous fight-or-flight response in the body." This is why it may be anxiogenic (anxiety-creating) for some patients.
Phentermine vs. Adderall
Patients are often curious about whether phentermine or Adderall is a better option for weight loss, given that Adderall has documented weight loss effects.
Adderall is an amphetamine, which is a class of stimulatory drugs that increase metabolic rate and can cause weight loss.
We do not recommend Adderall for weight loss, as this would be an "off-label" use of the drug. Adderall is not currently approved by the FDA for weight loss, but phentermine is.
For patients interested in pharmaceutical therapy for weight loss, we would recommend speaking with your doctor about phentermine rather than Adderall.
Phentermine Dosage
According to the drug's FDA label, phentermine is typically dosed at 37.5 milligrams (mg) once-daily.
In some patients, a dose of half of a tablet may be effective. This would provide 18.75 mg of phentermine.
Doctors may prescribe phentermine at a lower dose to reduce the risk of side effects. The FDA label even states that "dosage should be individualized to obtain an adequate response with the lowest effective dose."
Even if weight loss is slower at 18.75 mg daily, a doctor may recommend that a patient stay at that dose as long as consistent weight loss occurs, because it may be safer than a higher dose.
