Disclaimer: None of the information in this article constitutes medical advice, and is just the opinion of the writer(s) and published for informational purposes only. 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) in the U.S. for weight loss. It’s one of the most commonly-prescribed pharmaceutical treatments for obesity.
The full drug 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 called Adipex-P and Lomaira. These terms all refer to the same active drug ingredient so we'll use them interchangeably throughout this article.
In this article we’ll review medical studies on phentermine to provide our determination on whether it's safe and effective for weight loss. We'll also highlight side effects of the medication, and explain whether the brand-name versions are likely to be more effective.
Does Phentermine Work for 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 weight reduction effects of phentermine on an obese patient population. The researchers documented that after 14 weeks, nearly 90% of the patients taking phentermine lost 5% of body weight or more, and 50% of patients taking phentermine lost 10% of body weight or more. This would be categorized as a short-term trial, and these results are highly impressive.
These figures were significantly higher than the placebo group, of which 17% lost 5% of body weight or more, and 8.3% lost 10% of body weight or more.
A separate medical review analyzed the efficacy of long-term phentermine use; up to 24 months continuously. 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 6 clinical trials on phentermine. The authors noted that there was a statistically significant weight loss 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.
Phentermine 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 treatments. 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.
The 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 did carry a black box warning.
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.
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.
Should I Take Generic Phentermine or Branded Versions?
Patients are often confused about whether to take the branded or generic version of a drug, given that they contain the same active drug ingredient.
An extensive 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.
Given that generic drugs are often cheaper than branded drugs, we would recommend that patients speak with their doctor about phentermine rather than its branded alternatives.
How Does Phentermine Work?
Phentermine works primarily through appetite suppression. When patients take phentermine pills, their levels of a neurotransmitter called norepinephrine increase significantly. 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 by 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.
Is Phentermine Available Over-The-Counter (OTC)?
Phentermine is not available for purchase OTC, at least in the U.S., which means that the drug requires a prescription and cannot be purchased in a drugstore or online without one.
Phentermine is a pharmaceutical drug with a significant side effect profile, and this type of medication is typically not available OTC.
The drug can be purchased on different online retailers like Amazon, but a prescription is still required. Amazon has a service called Amazon Pharmacy that allows patients to contact pharmacists virtually.
We do not recommend purchasing phentermine from any online retailer that doesn't require a prescription, because the risk of fake pills is likely to be significantly higher.
OTC Weight Loss Options
There exist several over-the-counter (OTC) weight loss products that don't require a prescription, and have medical research backing. We are not suggesting that these products are as effective as phentermine; just that it may be worthwhile for an overweight patient to discuss these options with their doctor given their documented efficacy and lack of significant side effects.
We recommend dietary fiber as a safe and effective weight loss supplement.
An extensive medical review published in The Journal of Nutrition found that dietary fiber intake directly predicts weight loss when consumed at a high enough dose. It's zero-calorie plant matter that makes you feel full faster, and consume fewer calories overall.
The fiber supplement we recommend is SuperGut Fiber Mix. It contains a clean and effective formulation: a blend of three different types of unflavored dietary fiber and zero additive ingredients. It can be mixed into liquids or foods. Interested consumers can buy SuperGut fiber at this link.
We recommend using two fiber mixes per day, which provides 16 g of total fiber. This is within the effective fiber dosing range associated with the greatest weight loss outcomes in the above-linked study (8-29 additional grams per day).
Medium-chain triglyceride (MCT) oil is another dietary supplement which has been shown in clinical trials to cause weight loss.
MCT oil is quickly absorbed by the body and increases metabolic rate, which causes fat loss. A meta-study on MCT oil documented weight loss of 1.12 pounds over 10 weeks. This equates to a potential annualized weight loss of 5.84 pounds with MCT oil supplementation.
We recommend Bulletproof MCT Oil as our top MCT oil product, because it has a clean and effective formulation. The only ingredient is MCT oil derived from coconuts, and the product has no questionable additives. Interested consumers can buy Bulletproof MCT Oil at this link.
The effective dose range of MCT oil for weight loss (based on the medical review) is 1.7 g to 10 g per day. Bulletproof's MCT oil provides 14 g in one tablespoon, so around two-thirds of one tablespoon should be a maximally-effective dosage.
Phentermine User Reviews
Phentermine has been reviewed over 2,000 times on Drugs.com, which is a website that allows prescription drug users to publish reviews of their personal experience using a medication.
The average rating for phentermine is 8.7/10, which is one of the best average ratings for any medication we've reviewed.
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".
Phentermine Before-and-After Images
Patients are often curious about before-and-after images of weight loss medication from real users, because manufacturers rarely share this type of information on their website.
One of the most popular YouTube reviews of phentermine comes from a channel called "Norisbel Cayo" and the creator shares before-and-after images from one month of phentermine use. They claim the drug caused them to lose 20 lbs:
Phentermine and Alcohol
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 are honest with their doctor about any alcohol use, because this information 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 this potential interaction.
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 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 but not Adderall.