Phentermine History: Adipex-P & Ionomin. Fen-Phen Explained. Fenfluramine is Not Phentermine & Phentermine is Safe. 

Phentermine hydrochloride: 37.5 mg tablets
Phentermine is one of the oldest appetite suppressants still on the market. Phentermine was originally approved by the FDA in 1959. Phentermine, like many drugs of its era, was named as a variant of its much longer chemical name:  phenyl-tertiary-butylamine. Phentermine is the most widely prescribed weight loss drug in human history and is also one of the most commonly prescribed drugs period. Phentermine has stood the test of time and the test of numbers. As I indicated above, phentermine has been on the market in the US for over fifty years and it has been used around the world by tens of millions of human beings. Although phentermine, like all medications, caries some risks, those risks are very well understood and generally easily avoided through intelligent screening and prescription.

Better still, phentermine really works. And when I say (or write) that I mean that it REALLY works, that it is not merely effective but also efficacious and in case you aren't sure what the difference is between effectiveness and efficacy, I have provided a fairly long explanation for you. At any rate, phentermine causes weight loss. It works. It is pretty much the gold-standard among prescription appetite suppressants. It has been so for years and I suspect it shall thus remain, Qnexa notwithstanding.

The Many, Many Brand Names of Phentermine

Because of its vintage, phentermine's patent expired decades ago and since then it has been sold under dozens of generic trade names. Some of the most common of those include Ionomin, Fastin and Adipex-P. Basically, as with all drugs, the issues related to a particular brand of phentermine are several:

  1. Dosage: How much drug is in a pill or capsule (how many milligrams?)
  2. Release timing: does the pill or capsule dissolve as fast as a sugar cube in water and suddenly release its medication or is the drug slowly released like, say, pain in a sponge?
  3. Bioavailability: How much of the total drug that is inside the pill or capsule actually gets inside the blood stream? The goal of all drug manufacturers is to approach 100% bioavailability but for various practical reasons, that isn't always possible.
  4. Phentermine hydrochloride versus phentermine base: Phentermine can come as a salt (hydrochloride) or in its "pure" basic form. Ionomin contained phentermine base.
The point is that over the years, there have been a lot of different doses, release-formulations and bioavailabilities of phentermine and that point to this whole discussion is that sometimes I'll see a patient to who I dispense phentermine who tells me that the medication "feels" noting like it "used to" when that patient was prescribed it, say, decades ago. Usually the reasons for the differences are related to the issues that I have cited above.

Various Doses and Forms of Phentermine Over the Years

  • 8mg green tablets
  • 8mg pink tablets
  • 8mg white tablets
  • 15mg grey/yellow capsules containing 15 mg phentermine base
  • 18.75 grey/yellow capsules containing 18.75 mg phentermine hydrochloride
  • 30mg yellow capsules containing phentermine base (Ionomin)
  • 30mg yellow capsules contain phentermine hydrochloride
  • 30mg black capsules 
  • 30mg blue/white capsules
  • 30mg blue/clear capsules
  • 37.5mg green/clear capsules
  • 37.5mg red/clear capsules
  • 37.5mg white tablets with blue flecks (Adipex-P)
The point here is that phentermine has come in MANY sizes, doses, shapes and colors. Today, only a few of these remain.

Phentermine and Fen-Phen

Phentermine was indeed half of the fen-phen diet drug combination of the 1990s. This fact has caused (and still causes a lot of fear and confusion). I'd like to spend a little time trying explain matters.

Fen-Phen was a Nick-Name for "Fenfluramine and Phentermine"

It was easier to say "Fen-Phen" than "Fenfluramine and Phentermine". The combination became very popular in the early 1990s because of studies published by Dr. Micheal Weintraub which showed that the combination worked better than either drug alone. The fenfluramine and phentermine dose combination used in the studies became known as "The Weintraub Protocol" and became a huge "hit" around the country. What nobody knew at the time was the fenfluramine was a dangerous drug that should never have been on the market. The problem was that fenfluramine had been been approved as a single-agent appetite drug decades earlier but it had been a sales flop and so it was almost never prescribed...UNTIL fen-phen. And then all of a sudden, in just a couple years, fenfluramine went from being a drug taken by maybe a few thousand patients in the US to one taken by thirty million people. And THAT is when it became obvious that something was wrong. As probably everyone now knows, by the summer of 1997, it was clear that people taking fen-phen were at significant risk for developing heart valve damage.

Fenfluramine Alone or In Combination With Phentermine but NOT Phentermine Alone was the Danger in Fen-Phen

One of the biggest misconceptions about fen-phen is that the risk derived from the COMBINATION of the two drugs. Indeed, this WAS the concern initially, but since then (1997) it has become clear that the risk was ONLY related to the fenfluramine in fen-phen and was specifically NOT related to the phentermine.

Fenfluramine Caused Heart Valve Damage Because it Was a Serotonin Agonist and, it Turns Out, ALL Serotonin Agonists Cause Heart Valve Damage

A serotonin agonist is a drug that directly binds to a protein molecule found on the surface of heart cells (and other cells) that are called "serotonin receptors". I want to be clear here that a serotonin agonist like fenfluramine is NOT the same thing as a selective serotonin reuptake inhibitor like Prozac, Zoloft, Paxil and other antidepressant medications. We are talking about AGONISTS, not reuptake inhibitors. Anyway, it turns out that nearly all drugs that function as serotonin agonists cause cardiac fibrosis which, among other things, causes valve damage in the heart. Fenfluramine is a serotonin agonist. Fenfluramine causes cardiac fibrosis. Fenfluramine is unsafe. Phentermine is not a serotonin agonist. Phentermine has nothing to do with serotonin. Phentermine does NOT cause cardiac fibrosis. Phentermine IS safe.