Phentermine

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

Phentermine 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. Phentermine has not been reported an addictive potential which allows this agent to be classified under the Schedule IV drugs (low abuse potential).

Phentermine 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.

Phentermine
Uses
Dosage
Side Effect
Precautions
Interactions
Uses during Pregnancy
Uses during Breastfeeding
Accute Overdose
Food Interaction
Half Life
Volume of Distribution
Clearance
Interaction With other Medicine
Contradiction
Storage

Trade Name	Phentermine
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
Type	Capsule
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
Manufacturer
Available Country	United States
Last Updated:	September 19, 2023 at 7:00 am

Uses

Phentermine 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.

Phentermine 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.

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

How Phentermine works

Phentermine 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. Phentermine 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.

Toxicity

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.

Phentermine 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.

Phentermine Drug Interaction

Major: duloxetine, escitalopram, fluoxetine, bupropion, sertralineModerate: levothyroxine, topiramateUnknown: zolpidem, diphenhydramine, omega-3 polyunsaturated fatty acids, cyclobenzaprine, pregabalin, acetaminophen / hydrocodone, montelukast, cyanocobalamin, ascorbic acid, ergocalciferol, cholecalciferol, alprazolam, cetirizine

Phentermine Disease Interaction

Major: cardiovascular, glaucoma, agitation, cardiac disease, glaucoma, hypertension, liver disease, psychiatric disorders, pulmonary hypertension, substance abuse, ticsModerate: bipolar disorders, psychotic disorders, renal dysfunction, seizure disorders, diabetics, liver disease

Volume of Distribution

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

Elimination Route

Phentermine 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.