These two medications are appetite suppressants that are usually prescribed to control weight gain. Studies carried over a lengthy period have proved that they help to diminish appetite in different ways, but all have certain side effects that might cause adverse reactions in those taking these medications.
Fen-Phen was banned by US FDA long ago. See our top better alternatives below
The side effects of phentermine on the other hand were seen to cause nervousness, insomnia, irritability, dry mouth, dizziness, constipation, irregular heart beats and rise in blood pressure. Although taking it decreased the appetite, it too was seen to have the same negative reaction causing pulmonary Hypertension, life threatening complication.
What the clinical research says?
A new study carried out in July 1997 discovered that side effects of fen-phen included valvular heart disease as well as headache, nervousness, constipation and insomnia although it did bring about a decreased need for eating and made the patient feel good while taking the medication. It was thought at that time that a combination of these two medicines would be of benefit taking into account the different pharmacologic effects of the two drugs for better results and at the same time be able to minimize the negative effects considerably. [ref]
The FDA Ban on Fen phen
This combination however did not receive the approval from the Food and Drug Administration and was subsequently withdrawn in 1997. The side effects of fen-phen even though thought to be mild and harmless at the initial stages, were considered a risk with longer use and was in danger of accelerating its harmful effect by causing serious complications resulting in debility requiring surgery and intensive therapy and could be even fatal in the end.