Fen-Phen is known to cause harmful and fatal heart and lung damage. The most common heart damage caused by Fen-Phen is valvular heart damage, but Fen-Phen also causes Primary Pulmonary Hypertension:

Valvular Heart Damage
The most common valvular damages caused by Fen-Phen are Mitral Valve Regurgitation or Aortic Valve Regurgitation. Malfunction of one of the four valves within the heart can cause problems that affect the entire circulatory system. A leaky valve does not close all the way, allowing some blood to flow backward as the heart contracts. This backward flow decreases the amount of oxygen the heart can deliver to the tissues with each beat. Symptoms of heart valve disease include shortness of breath, swelling in the legs, chest pain and heart palpitations.

Primary Pulmonary Hypertension (PPH)
Primary Pulmonary Hypertension is a rare and fatal lung disease that narrows the blood vessels of the lungs and eventually leads to heart failure. Studies have shown that taking appetite suppressants, including Fen-Phen greatly increases the risk of developing this disease. There is no cure for PPH, and there are no medications that will cure or halt the progression of the disease. The median period of survival is three years after diagnosis.