Phendimetrazine tartrate is an anorectic (appetite suppressor) supplement designed to be used short-term to treat obesity. It is available only by prescription, and is generally (though not exclusively) prescribed for those with a BMI of 30+. 
The primary brand name for Phendimetrazine is Bontril, and that is the brand we’re reviewing, but it also goes by many other names—Adipost, Melfiat, and Plegine are just a few. Bontril is made by Valeant Pharmaceuticals, based out of New Jersey. 
Bontril comes in two forms: 35 mg immediate-release tablets, and 105 mg extended-release capsules. The cost ranges from $15.00-$30.00 for 90 pills, and can be obtained with a prescription at most pharmacies around the country.
Essentially, Bontril offers two things: appetite suppressant and energy boost. It doesn’t come with a lot of bells-and-whistles claims. This is pretty much the norm with the heavy-duty, prescription-only weight loss aids, which are only meant to be taken in short spans under the close supervision of a health care professional in combination with a strict diet/exercise regimen. So not really a lot of room for “miracle pill” marketing.
Side effects mentioned on WebMD.com include: “A raise in blood pressure, dizziness, dry mouth, difficulty sleeping, irritability, nausea, vomiting, diarrhea, or constipation.” 
Bontril and other phendimetrazine drugs are on the Mayo Clinic’s radar, and the potential side effects are clearly listed. There are rare incidents of sensory issues (seeing, hearing, feeling things that aren’t there) and severe mental changes. More commonly reported, though Mayo doesn’t specify frequency, are things like anxiety, dizziness, dry mouth, heartbeat racing or pounding, headache, nausea, swelling or shaking in the hands or feet, increased urination, and others. 
The 35 mg immediate-release tablets: 35 mg Phendimetrazine tartrate, Compressible Sugar, Confectioner’s Sugar, D&C Yellow #10, FD&C Blue #1, FD&C Yellow #6, Isopropyl Alcohol, Lactose Anhydrous, Magnesium Stearate, Microcrystalline Cellulose, Povidone, Purified Water, Sodium Starch Glycolate.
The 105 mg extended-release capsules: 105 mg Phendimetrazine tartrate, FD&C Yellow No. 6, and whatever is used to make the capsule (it’s not specified).
The Science (or Lack Thereof) Behind Bontril
Given that the Mayo Clinic, the FDA, and several other reputable organizations speak of Phendimetrazine tartrate as pretty much a given in the field of appetite suppressants, even with the list of side effects, one generally assumes there’s science to back it up. It is definitely not recommended for people under 12 years of age (pediatric obesity is treated as a long-term chronic condition— Phendimetrazine tartrate is a short-term medication), nor for pregnant or breastfeeding women.
I didn’t find a lot of specific studies on it, though. There are summaries of the results of multiple studies, in particular from DailyMed.com at the US National Library of Medicine. But the summary also points out the problems in doing such studies:
Adult obese subjects instructed in dietary management and treated with anorectic drugs, lose more weight on the average than those treated with placebo and diet, as determined in relatively short term clinical trials.
The magnitude of increased weight loss of drug-treated patients over placebo-treated patients is only a fraction of a pound a week. The rate of weight loss is greatest in the first weeks of therapy for both drug and placebo subjects and tends to decrease in succeeding weeks. The possible origin of the increased weight loss due to the various drug effects is not established. The amount of weight loss associated with the use of an anorectic drug varies from trial to trial, and the increased weight loss appears to be related in part to variables other than the drug prescribed, such as the physician investigator, the population treated, and the diet prescribed. Studies do not permit conclusions as to the relative importance of the drug and non-drug factors on weight loss.
The natural history of obesity is measured in years, whereas the studies cited are restricted to a few weeks duration; thus, the total impact of drug-induced weight loss over that of diet alone must be considered clinically limited.  (emphasis added)
In short, Phendimetrazine tartrate seems to work; researchers just haven’t been able to determine exactly why it works.
Word on the Street About Bontril
First and foremost to know: Phendimetrazine tartrate is in that group of weight-loss aids classified as amphetamine-like. The FDA and US Drug Enforcement Agency (DEA) have classified Phendimetrazine as a Schedule 3 (III) controlled substance/drug. “The drug has a potential for abuse less than the drugs in schedules 1 and 2. The drug has a currently accepted medical use in treatment in the United States. Abuse of the drug may lead to moderate or low physical dependence or high psychological dependence.” 
This drug is not meant for someone looking to lose 10-20 pounds, but rather for someone who is categorized as obese and needing to shed a lot of weight (50+ pounds) for many health-related reasons. As mentioned above, it is not suitable for extended use. And the reviews from users are mixed.
“Momofmy4babies,” 2015: “I only took Bontril 105mg for 2 months. I started out at 250lbs!!! I went to buy New clothes and was in a size 18, so depressing! I started out on Bontril 35mg, that did nothing for me so they bumped it up. I did stop drinking cold drinks, and only water. I did not even drink tea. I ate a high protein diet, beef jerky, cheese, and yogurt. They told me 1200 calories but that felt impossible! I did exercise the first month but slacked in the second. I lost 50 pounds total in those 2 months. There after I did not gain my appetite back and got all the way down to 162 pounds. This medicine is absolutely amazing! Its been over 2 years and I just starting gaining some of the weight back due to the depo shot, I will definalty be going back to Bont” 
“Joey” said, “I started taking this on Feb 9, 2017. It is now April 8, 2017. Two months on this medicine I have lost 18 pounds with 11 pounds going in the first 4 weeks. At first I only took half what I was prescribed which is 2 pills three times a day.” 
“Dana” said, “I have been taking Phendimetrazine 35mg 1 pill then increased over a 2 month period to, 2 pills 3x a day for about 4 months. I developed symptoms of hoarseness, sensation of a lump in my throat, choking spells daily, excessive phlegm, swallowing issues, chest pains, severe upper stomach pains, breathing problems.” 
Every supplement affects every individual differently. Gender, age, sometimes race, pre-existing medical conditions, and individual body chemistry—as unique as a fingerprint—can affect how a supplement works on you. That’s why supplements always add that “individual results may vary” disclaimer, and it’s why reviews for a drug or supplement can be all over the spectrum. Only you and your doctor can figure out how it works for you, so always be forthcoming and transparent with your doctor about side effects you experience, as well as if the substance doesn’t seem to be doing anything for you.
The Bottom Line
Is Bontril Worth A Try?
Depends. Discuss this one with your doctor. It certainly seems to work in a lot of cases, but researchers aren’t quite sure why, the side effects can be harsh, and there is a chance for dependence if it’s not taken properly. There are several different chemical substances in this anorectic class, and if this one isn’t right for you, another might be.
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*Individual results will vary.