This is a US colloquial term for two drugs, phentermine and fenfluramine, used in combination as slimming pills. (As well as phin-fen, the forms fen-phen and phen/fen are also common.)
Phentermine, sold as Adipex, Fastin and Ionamin among other trade names, is an appetite suppressant and a mild amphetamine-like anti-depressant that can help to make takers feel more energetic, and so burn off calories. Fenfluramine, sold under the name Pondimin, acts as an appetite suppressant by increasing the level of serotonin, which makes the body feel satiated.
Their use has become common in the US since it was discovered in 1992 that taking them together enhanced the anorectic (appetite reducing) action while reducing some side effects. More than 18 million prescriptions were reportedly issued in 1996, despite the treatment not having received formal regulatory approval because no studies have been carried out on possible interactions between them (a variant drug dexfenfluramine, sold as Redux, has been approved by the US Food and Drug Administration, the first slimming pill to do so for 20 years).
In August the New England Journal of Medicine reported a study by a team at the Mayo Clinic which found that these appetite suppressants were in some cases causing heart problems and pulmonary hypertension (an increased resistance to the flow of blood through the lungs) which have caused deaths. In October, the FDA advised patients to stop taking dexfenfluramine and fenfluramine and their manufacturers withdrew them from the market.