Fentermina Uses, Dosage, Side Effects, Food Interaction and all others data.

Fentermina is a sympathomimetic amine anorectic agent and it was introduced in 1959 as part of an anti-obesity combination drug. It is chemically related to amphetamine and it is commonly referred to as an atypical amphetamine. Fentermina has not been reported an addictive potential which allows this agent to be classified under the Schedule IV drugs (low abuse potential).

Fentermina was FDA approved for short-term weight management in 1959 and it became widely used in 1960. This initial product, formed by the combination of phentermine with fenfluramine and dexfenfluramine was discontinued after finding several reports of abnormal valves in nearly 30% of the consumers. Later on, phentermine was approved alone and in combination with topiramate in 2012 as a new alternative that required lower doses of phentermine to obtain the desired effect.

It is reported that the main mechanism of action of phentermine is the generation of appetite suppression, maybe due to the increase in leptin, but it is considered that other mechanisms should be involved. Some reports have indicated that the weight loss effect is mainly due to the increase in resting energy expenditure.

Trade Name Fentermina
Availability Prescription only
Generic Phentermine
Phentermine Other Names Fentermina, Phentermine, Phentermine resin, Phenterminum
Related Drugs Victoza, semaglutide, Wegovy, Saxenda, liraglutide, methamphetamine, Contrave, Qsymia, oxandrolone, Alli
Formula C10H15N
Weight Average: 149.2328
Monoisotopic: 149.120449485
Protein binding

The protein binding of phentermine is determined to be of 17.5%.

Groups Approved, Illicit
Therapeutic Class
Available Country
Last Updated: September 19, 2023 at 7:00 am


Fentermina is a sympathomimetic anorectic agent used as a short-term adjunct therapy that is included in a regimen of weight reduction in cases of exogenous obesity.

Fentermina is indicated, alone or in combination with topiramate, as a short-term adjunct, not pass a few weeks, in a regimen of weight reduction based on exercise, behavioral modifications and caloric restriction in the management of exogenous obesity for patients with an initial body mass index (BMI) greater than 30 kg/m2 or greater than 27 kg/m2 in presence of other risk factors such as controller hypertension, diabetes or hyperlipidemia.

Exogenous obesity is considered when the overweight is caused by consuming more food than the person activity level warrants. This condition commonly causes an increase in fat storage. It is an epidemic condition in the United States where over two-thirds of adults are overweight or obese and one in three Americans is obese. In the world, the incidence of obesity has nearly doubled.

Fentermina is also used to associated treatment for these conditions: BMI >30 kg/m2

How Fentermina works

Fentermina is an indirect-acting sympathomimetic agent that acts by releasing noradrenaline from the presynaptic vesicles in the lateral hypothalamus. This increase in noradrenaline concentration in the synaptic cleft results in the stimulation of beta2-adrenergic receptors. Fentermina is classified as an indirect sympathomimetic due to the increase in the level of norepinephrine, dopamine and its indirect effect towards serotonin. Some reports have indicated that phentermine inhibits the neuropeptide Y which is a principal signaling pathway for the induction of hunger. This combined effect produces a continuous flight-or-fight response in the body which reduces the hunger signal as this state is on the immediate need for energy.

Lastly, some reports have indicated that phentermine is a weak inhibitor of monoamine oxidase but this mechanism does not tend to produce a clinically significant response.


The reported LD50 after oral administration of phentermine in rats is reported to be of 151 mg/kg. Reports of acute overdose include restlessness, tremors, hyperreflexia, rapid respiration, confusion, assaultiveness, hallucinations and panic state followed by fatigue, and depression. In the cardiovascular system, there are reports of tachycardia, arrhythmia, hypertension, hypotension, circulatory collapse. In the GI tract, there are symptoms of nausea, vomiting, diarrhea and abdominal cramps. The management of acute overdosage includes symptomatic treatment as well as lavage and sedation with barbiturates.

On the other hand, chronic overdosage is marked by dermatoses, insomnia, irritability, hyperactivity and personality changes. In severe cases, it can derive into a schizophrenia-like psychosis.

Studies regarding the carcinogenic potential have not been performed. On the case of mutagenic assays, phentermine was shown to not be mutagenic nor clastogenic.

Food Interaction

  • Limit caffeine intake.
  • Take with or without food. The absorption is unaffected by food.

[Moderate] GENERALLY AVOID: Alcohol may potentiate the central nervous system and cardiovascular effects of centrally-acting appetite suppressants.

In one study, concurrent administration of methamphetamine (30 mg intravenously) and ethanol (1 gm

This increases cardiac work and myocardial oxygen consumption, which may lead to more adverse cardiovascular effects than either agent alone.

Subjective effects of ethanol were diminished in the eight study subjects, but those of methamphetamine were not affected.

The pharmacokinetics of methamphetamine were also unaffected except for a decrease in the apparent volume of distribution at steady state.

MANAGEMENT: Concomitant use of centrally-acting appetite suppressants and alcohol should be avoided if possible, especially in patients with a history of cardiovascular disease.

Patients should be counselled to avoid hazardous activities requiring complete mental alertness and motor coordination until they know how these agents affect them, and to notify their physician if they experience excessive or prolonged CNS effects that interfere with their normal activities.

Fentermina Hypertension interaction

[Major] The use of CNS stimulants is contraindicated in patients with significant cardiovascular impairment such as uncompensated heart failure, severe coronary disease, severe hypertension (including that associated with hyperthyroidism or pheochromocytoma), cardiac structural abnormalities, serious arrhythmias, etc.

Sudden death has been reported in adults and children taking CNS stimulant treatment.

Additionally, stroke, myocardial infarction, chest pain, syncope, arrhythmias and other symptoms have been reported in adults under treatment.

A careful assessment of the cardiovascular status should be done in patients being considered for treatment.

This includes family history, physical exam and further cardiac evaluation (EKG and echocardiogram).

Patients who develop symptoms should have a detailed cardiac evaluation and if needed, treatment should be suspended.

Hypertension interaction

[Major] CNS stimulant medications have shown to increase blood pressure, and their use might be contraindicated in patients with severe hypertension.

Caution should be used when administering to patients with preexisting high blood pressure and other cardiovascular conditions.

All patients under treatment should be regularly monitored for changes in blood pressure and heart rate.

Volume of Distribution

The reported volume of distribution for phentermine is reported to be of 5 L/kg.

Elimination Route

Fentermina shows a dose-dependent pharmacokinetic profile. After oral administration of a dose of 15 mg, the maximal concentration was achieved after 6 hours and its bioavailability was not affected by the consumption of high-fat meals. The reported plasma concentration at steady-state is of around 200 ng/ml as observed in clinical trials.

Half Life

The mean terminal half-life of phentermine is reported to be of approximately 20 hours. In conditions where there is acidic urine (pH 9


The reported clearance when administered orally is 8.79 L/h as observed in pharmacokinetic population studies.

Elimination Route

Fentermina is excreted mainly in the urine from which about 70-80% of the administered dose can be found as the unchanged drug.

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