"Phentermine and diethylpropion are central stimulants; they are not recommended for the treatment of obesity. Phentermine has been associated with a risk of pulmonary hypertension."so I'm not sure that it's currently prescribed in the UK at least, or perhaps only in the very short term. I couldn't find any reference to phentermine (or phentemine) as a drug product on the electronic Medicines Compendium, the only references are where it's not recommended in combination with another drug.
Basically phentermine appears to be a not very nice drug that isn't recommended for weight loss in the UK but it is prescribed in the US (see bit on Qsymia below) - I presume this means that people are using it under the care of a doctor, they're not just buying it off the internet.
"Phentemine375 uses a combination of cyclic AMP enzyme boosters such as , 1,3,7-Trimethylxanthine, and a Sympathomimetic Amine, LongJack Tongkate ALI,Capsaicin-1.12, l-carnitine to get the job done. Each of the five main ingredients are highly refined and produced in a pharmaceutical environment. This provides you with the strongest and purist product humanly possible to produce."