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Phentermine (phenyl tertiary butylamine, C10H15N)



Each Duromine capsule contains phentermine as an ion-exchange resin complex equivalent to 15 or 30 mg phentermine depending on strength. The ion-exchange resin is quite stable, highly insoluble and without pharmacological effect until it reacts with cations (hydrogen, potassium, sodium etc) present in the gastrointestinal fluids. Phentermine is then released from the resin complex at a rate dependent on the total concentration of these cations. Since this concentration is fairly constant throughout the entire gastrointestinal tract, continuous and controlled ionic release occurs over a 10 to 14 hour period.

Pharmacodynamic properties
Phentermine is a sympathomimetic amine with significant anorectic activity in animal models. Its appetite suppressant effect is generally considered to be exerted through the hypothalamus, but it is not certain that this is the only effect related to weight loss. Phentermine has major effects on the dopaminergic and noradrenergic nervous systems. The cardiovascular effects include a pressor response and increase in heart rate and force of contraction.

Pharmacokinetic properties
Absorption of phentermine is almost complete. The rate of absorption from the resin complex is significantly slower than that from the hydrochloride salt resulting in a lower and later peak blood level. Phentermine is readily absorbed from the gastro-intestinal tract. Following an oral dose of phentermine capsule, one study demonstrated urinary excretion of unchanged drug ranging from 62.7% to 84.8% in 72 hours. The remainder is metabolised in the liver. The half-life of phentermine is about 25 hours. In one study in volunteers acidification of the urine reduced the half-life to 7 - 8 hours.

Duromine is an anorectic agent indicated in the management of obesity as a short-term adjunct in a medically monitored comprehensive regimen of weight reduction based, for example, on exercise, diet (caloric/kilojoule restriction) and behaviour modification in obese patients with a body mass index (BMI) of 30 kg/m2 or greater. The treatment with Duromine can be initiated in overweight patients with a lower BMI (25 to 29.9 kg/m2) which increases the risk of morbidity from a number of disorders. Secondary organic causes of obesity should be excluded by diagnosis before prescribing this agent.

Pulmonary artery hypertension; existing heart valve abnormalities or heart murmurs; moderate to severe arterial hypertension; cerebra-vascular disease; severe cardiac disease including arrhythmias, advanced arteriosclerosis; known hypersensitivity to sympathomimetic drugs; hyperthyroidism; agitated states or a history of psychiatric illnesses including anorexia nervosa and depression; glaucoma; history of drug/alcohol abuse or dependence; concomitant treatment with Monoamine Oxidase (MAO) Inhibitors or within 14 days following their administration.

Special Precautions
Duromine capsules are indicated only as short-term monotherapy for the management of exogenous obesity. The safety and efficacy of combination therapy with phentermine and any other drug products for weight loss have not been established. Therefore, coadministration of drug products for weight loss is not recommended.

Since the selective serotonin reuptake inhibitors (e.g., fluoxetine, sertraline, paroxetine), ergot-like drugs and clomipramine affect serotonin disposition there remains a theoretical risk that combination of these agents with phentermine may also be associated with cardiac valvular disease and is not recommended. There is no direct scientific evidence to confirm this theory.


Valvular Heart Disease: Serious regurgitate cardiac valvular disease, primarily affecting the mitral, aortic and/or tricuspid valves, has been reported in otherwise healthy persons who had taken a combination of fenfluramine or dexfenfluramine with phentermine for weight loss. The aetiology of these valvulopathies has not been established and their course in individuals after the drugs are stopped is not known. There have been no reported cases to date of valvular heart disease occurring with the use of phentermine alone.


Primary Pulmonary Hypertension (PPH): Cases of severe, sometime fatal primary pulmonary hypertension, have been reported in patients who have received anorectics. In a case-control epidemiological study, the duration of treatment with anorectic agents, not including phentermine, beyond three months significantly increases the risk of PPH. However, patients treated with phentermine require medical review at least every 3 months (Refer to "Dosage and Administration"). PPH has been reported in patients receiving fenfluramine/dexfenfluramine combined with phentermine. The possibility of an association between PPH and the use of phentermine alone cannot be ruled out; there have been very rare cases of PPH in patients who reportedly have taken phentermine alone. The initial symptom of PPH is usually dyspnoea. Other early symptoms include: angina pectoris, syncope, lower extremity oedema or the unexplained onset or aggravation of diminished exercise tolerance. Under these circumstances, treatment should be immediately discontinued and the patient referred to a specialist unit for investigation.

Use with caution in the following circumstances
Duromine should be used with caution in patients with mild hypertension. In the first days of treatment, determine that there is no loss of blood pressure control. In patients receiving Duromine, response to insulin and oral hypoglycaemic agents may vary due to alterations in dietary regimes. This should be kept in mind if Duromine is used in diabetic patients.

Duromine may impair the ability to perform activities requiring mental alertness, such as driving and operating machinery, and patients therefore should be cautioned accordingly. Inappropriate use has been reported with similar drugs and the possibility of this occurrence should be considered with Duromine.

Cardiovascular and cerebrovascular events have rarely been reported, mainly in association with rapid weight loss. Weight loss should be gradual and controlled in obese patients undergoing treatment with Duromine. Duromine should be used with caution in patients with established coronary artery disease. A single case of exacerbation of angina pectoris in a patient with established coronary artery disease has been reported. Duromine should be used with caution in patients receiving psychotrophic drugs, including sedatives and agents with sympathomimetic activity. Duromine should be used with caution in epileptic patients. Duromine should be used with caution in patients receiving anti-hypertensive agents.


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