Phentermine is supposed to be used only by obese people with a strictly physician-dictated program that includes a healthy low-calorie diet and regular exercise. And it’s only to be prescribed for a short period of time.
I have noted that some people who claim to have been prescribed phentermine have been on it for many months; it may be that they were off and on, off and on, in cycles under doctor supervision so as to avoid dependency. Because bottom line: this is an addictive amphetamine-like central nervous system stimulant and a very strong one. It dispatches appetite and that’s why people take it; so they won’t eat. 
But that’s part of the problem, because if you use—and ultimately, potentially abuse—phentermine without a prescription and medically-supervised plan, the risks are many. Forget the fact that you’ll probably lose weight; you could very well permanently damage your body and your life in the process. Let’s take a closer look.
This drug is a stimulant that increases energy and crushes appetite. It is what’s known as a sympatho-mimetic amine and is in the same drug class as amphetamines: methamphetamine, ephedrine, pseudoephedrine. You get the picture: Speed. Uppers. The Phentermine website describes itself as the “official website for generic phentermine with all the information you need and a large community of members to help you reach your goal.” 
It’s pretty unclear who is doing the selling, because according to the U.S. Drug Enforcement Agency (DEA), phentermine—under brand names include Ionamin, Fastin and Adipex—is a diet pill created to replace amphetamines and is one of the federal Controlled Substance Act (CSA) drugs.  
In other words, it is illegal unless used for the prescribed medical purpose. The DEA says the amphetamine-like phentermine is “the most widely prescribed and most frequently encountered on the illicit market.” 
Phentermine used to be combined with fenfluramine—remember Phen-Fen? Fenfluramine was banned because it caused heart attacks.
On the Phentermine site, you can choose from three phentermine drugs: Phentermine 37.5, Adipex 37.5, or PhenCaps. According to the site, Phen-Caps do not require a prescription. The others presumably do (and we know they do), but you’re still able—after repeatedly clicking on the Phentermine or Adipex images—to end up at a checkout where you can purchase these diet pills without a prescription—or at least as far as I got, because I did not register or submit payment information. 
I caution you against purchasing these. This review is less a “review” than a set of facts about Phentermine used as a weight loss aid, based on my research.
A quick note about PhenCaps: these capsules are packed with unproven and potentially not-safe chemical compounds and plant extracts that may or may not be effective at appetite suppression, but who knows what’s even inside them? There’s no regulation, since they are considered dietary supplements. They also have caffeine, a laxative, and a type of synthetic phentermine alkalid that is also pretty sinister. Just don’t bother.
So, bottom line: Phentermine claims it will snuff out hunger and give you lots of energy. You know, because it’s like speed, basically. So the claim you’ll lose weight isn’t really far off. But at what cost? Because if you are not doing this medication under medical supervision, you’re fooling yourself. You can—and may very likely—end up being addicted. No weight loss is worth that.
Phentermine 37.5 is phentermine hydrochloride, and there’s 37.5 milligrams of it in each pill. There is no higher dose. Phentermine hydrochloride, by the website’s own admission, is “similar in structure to an amphetamine.” 
Stop right there. Why would you take this? Ask yourself. Unless you live under a rock, you by now know that amphetamines are dangerous drugs and highly addictive. Yes, the pill works by affecting parts of the central nervous system to crush hunger and make you zoom, presumably to work your butt off exercising. That’s the ideal, the site says.
The Science (or Lack Thereof) Behind Phentermine
So by now you know that Phentermine is to be used for a limited period of time to speed weight loss in very obese people who are exercising and eating a low-calorie diet. Phentermine is in a class of medications called anorectics that, in part, somehow chemically cause a severe decrease in appetite. So it works because you don’t want to eat. Pretty simple. 
A federally-regulated drug, you must have a prescription for it. Period. If you buy it off of a website, that’s not the way to do it.
The National Institutes of Health (NIH) agree that it does what it claims but should only be used “as part of a short-term plan, along with a low calorie diet, for weight reduction.” The NIH says this stimulant is used in obese patients who have not been able to lose weight with diet and exercise alone, but cautions it can be habit-forming. 
If you have been taking Phentermine and suddenly stop you will suffer from withdrawal symptoms, even if you’re doing it legally as prescribed by your doctor. WebMD says:
Do not stop using this medicine suddenly. Your doctor will need to slowly decrease your dose before you stop it completely.  (emphasis added)
Withdrawal symptoms can be debilitating: the physical ones alone can include intense and excessive hunger, physical instability, dehydration, shaking, tremors, gastrointestinal problems, tachycardia, and even heart attack. Then there are the psychological symptoms of withdrawal, and they can be even worse: they include intense irritability, extreme and violent mood swings, aggression, anxiety, suicidal ideas, hallucinations, and psychosis.
Just taking Phentermine, let alone stopping it, can include side effects like: dizziness, insomnia, vomiting, diarrhea, constipation, increased blood pressure, fast heartbeat, and a laundry list of other unpleasantness. WebMD again:
“Though it helps many people, this medication may sometimes cause addiction.” 
This is perhaps an understatement. Need I say more?
Word on the Street about Phentermine
The only Phentermine reviews I will quote come from the authoritative medical site WebMD.com. 
There are more than 2000 reviews, and the majority of the ones I read—around 200—seemed to be from people who were actually prescribed the drug by a physician for obesity. As I have said repeatedly, do not take this unless your doctor has prescribed it. Hundreds, indeed thousands, of people who reviewed this drug gave it an overall rating of 4.8 stars for effectiveness. There was no shortage of people who complained about the withdrawal symptoms and the side effects, but the majority were happy with the weight loss, and most were either seriously overweight or morbidly obese.
The story reviewer “Babe” shared is heartbreaking:
I had it over 15 years on and off. When I first started, I felt very good, and everything looked very positive. My heart was racing all the day. I couldn’t go sleep until 2:00 am in the morning. I lost about 30 pounds with 2 months. I was a very nice person, and I became angry all the time. I didn’t realize that I was getting meaner and meaner until my brother pointed it out to me. I was on the pill for 7 days, and stopped for 4-5 days, then back on for 7 days, on and off. Last year summer, I was sitting in my office chair, all a sudden I had a major panic attack right after I took the pill. I got cold sweat, couldn’t catch air, and rushed to the front office to screamed for help. I started to cry, then just realized it might be the reason triggered the attack, so I ran into the restroom and induced vomiting. The next 5 days, I had non-stop panic attacks. I was in and out ER, visited my Dr’s office, and finally it stopped. I stopped taking it almost a year now. I just don’t feel the same any more. It ruined my health, and my relationships with my love ones.” 
But “Craunte,” a woman around 40, could not be happier.
I have been overweight for my entire adult life. I have been on the medication now for 6 months and have lost 70 pounds. I still have 70 more to go. In addition to eating right and exercise this has been amazing!! I have my blood pressure monitored every other week and am happy to say has been normal every time. Would recommend this to anyone wanting to lose weight, just be sure your doctor monitors your blood pressure closely. 
And you wonder and worry about someone like “Grma,” a middle-aged woman who has recently started on the medication and appears concerned.
I have only been on this medication 5 days. I have been having moderate headaches and some anxiety feelings. Trying to decide if I should see if I adjust to it or stop taking it. 
The Bottom Line: Is Phentermine Worth a Try?
Unless your doctor has prescribed it, ab-so-lute-ly not. I don’t know how else to express the importance of making sure that you understand: if you are obese and nothing has worked, and your doctor is on board, and you are careful and conscious of the dangers and risks, then yes, you may be able to lose a lot of weight. But only under the circumstances I just described. Do not buy Phentermine online and use it thinking you know what you are doing.