by egpat 22 May 2024
Phentermine is classified as an anorexiant. These are the categories of drugs that are going to reduce appetite. That's why phentermine is used in the treatment of exogenous obesity, which is related to excessive food intake. This enhanced appetite can be controlled by phentermine. This drug is actually sympathomimetic, which is going to increase sympathetic activity.
Structurally, phentermine is related to amphetamine, which is a CNS stimulant as well as sympathomimetic. This amphetamine is classified as indirectly acting sympathomimetic. So phentermine also indirectly increases sympathetic activity. The name of the drug gives some structural information. The suffix -termine indicates this has a tertiary amine group. The prefix phen- indicates it designed the phenyl group.
So phentermine is a phenyl group containing a tertiary amine derivative that is structurally related to amphetamine, and this drug can reduce appetite. That's why it is classified as anorexiant, and particularly, this drug can be combined with other drugs such as topiramate, which is an antiepileptic.
However, this combination can also be used to control the appetite in order to manage obesity in patients. So phentermine is used in patients with a body mass index greater than 30 kg per square meter. This BMI can be measured as a ratio of weight in kg to height in square meters. When this BMI is greater than or equal to 30 kg per square meter, it indicates overweight. In such conditions, phentermine can be used to control the appetite.
However, in patients with any risk factors such as hyperglycemia, resulting in diabetes or any controlled hypertension, or increased levels of lipids resulting in hyperlipidemia, in all such conditions when the BMI is greater than or equal to 27 kg per square meter, the patient may have more risk for cardiovascular complications, so in those patients, again, phentermine can be used.
Even though the exact mechanism is not known, the sympathetic system plays an important role in controlling the appetite in the hypothalamus. Sympathetic neurons coming from the hypothalamus will store norepinephrine. When a stimulus reaches the presynaptic neurons, calcium can enter the presynaptic neurons, resulting in calcium-mediated exocytosis. This results in the release of norepinephrine, which acts on postsynaptic adrenergic receptors. This produces an excitation in the postsynaptic neurons, and this signal is going to be transmitted to the colon, where the appetite is going to be controlled.
However, the action of norepinephrine on appetite control depends on how much time it is present within the synaptic cleft. The released norepinephrine can be taken back into the nerve terminal by the norepinephrine transporter, which is expressed on the presynaptic neurons. This reduces its action.
Phentermine can increase sympathetic activity. It can enter the presynaptic neurons through these norepinephrine transporters, where it acts on the synaptic vesicles. It displaces the norepinephrine from storage vesicles, which can be leaked out of the membrane through the same transporter. In this way, phentermine can increase the release of norepinephrine into the synaptic cleft.
Now, because of the increased availability of norepinephrine, it increases sympathetic stimulation, resulting in increased control of appetite. So this is one possible mechanism of phentermine and how it can control the appetite. But this drug can also control other mediators, like neuropeptide Y, which again controls the appetite.
Phentermine, when given with alcohol, can increase sedation in patients. So that's why, in patients who are working with machinery or driving vehicles, this phentermine should be carefully given, and particularly when it is combined with alcohol, it can increase the sedation in the patients.
Similarly, phentermine, being a sympathomimetic, can increase sympathetic activity. So it can increase the blood pressure, resulting in hypertension in patients. So care should be taken with patients who already have elevated blood pressure. This drug further increases the blood pressure, resulting in hypertension in the patients.
When this drug is combined with other drugs such as fenfluramine and dexfenfluramine, the latter being a dextro isomer of fenfluramine, it can increase one of the conditions, valvulopathy. It can damage the heart valves. Particularly, it can affect the mitral valve, the aortic valve, and even the tricuspid valves, so that the pumping efficiency of the heart is going to be reduced. So this combination should be carefully given to patients with any pre-existing cardiovascular disorders.
This combination can also affect the lungs, where it can increase the blood pressure, resulting in primary pulmonary hypertension (PPH). Due to elevated blood pressure, it can produce a few of the symptoms, such as dyspnea (difficulty breathing), syncope (fainting), and even affect the heart, resulting in angina. It can also increase the edema in the patients. So all these are observed because of pulmonary hypertension produced by phentermine, particularly when it is combined with drugs such as fenfluramine and dexfenfluramine.
The important side effect of phentermine is on blood pressure. It can increase the blood pressure, resulting in hypertension. It can also produce some palpitations (awareness of the heartbeat), tachycardia, and an increased heart rate. It can produce some central side effects, such as dizziness, insomnia, and lack of sleep.
That's why this drug should not be given after the evening because it produces some insomnia in the patients. So this drug is preferred to be taken in the morning before breakfast in order to reduce any insomnia in the patients. It can also produce some euphoria, which may result in some addiction. Other side effects, including dry mouth, headache, constipation, and urticaria, can also be observed with this phentermine.
This drug is available as a tablet as well as a capsule. It is available in three strengths, such as 15 mg, 30 mg, and 37.5 mg. So here, phentermine is present as phentermine hydrochloride, which is not equivalent to phentermine. 15 mg of phentermine hydrochloride is equivalent to 12 mg of plain phentermine. Similarly, 30 mg of salt is equivalent to 24 mg of the base, and 37.5 mg is equivalent to 30 mg of phentermine.
However, this drug is always available in hydrochloride form. So it is available in three strengths: 15 mg, 30 mg, and 37.5 mg. The dose of the drug should be given at the lowest dose possible, and it is individualized based on the patient. Generally, the initial dose is started at 37.5 mg of phentermine hydrochloride. Now, this 37.5 mg of phentermine hydrochloride is equivalent to 30 mg of plain phentermine.
So the initial dose is started at 37.5 mg. But the dose may also be given as a half dose in patients with any risk factors such as renal dysfunction or hepatic dysfunction, and this drug should be given before breakfast, particularly one or two hours before breakfast, in order to avoid any gastrointestinal side effects or any insomnia developed in the patients.
Phentermine is an anorexiant that reduces appetite. That's why it is given to patients for the management of obesity, where the body mass index is greater than or equal to 30 kg per square meter. But in patients with any risk factors such as hyperglycemia, hypertension, and hyperlipidemia, even if the body mass index is greater than 27 kg per square meter, this drug can be prescribed.
Generally, phentermine is combined with other drugs such as topiramate, which is an antiepileptic agent that again controls the appetite; therefore, it is used in the management of obesity. Being a sympathomimetic, this drug can increase the blood pressure in patients, and it can also produce some euphoria and insomnia. That's why this drug should not be given after the evening in order to reduce any insomnia in the patients.
This drug is available as a salt form, phentermine hydrochloride, in three strengths: 15 mg, 30 mg, and 37.5 mg. The 37.5 mg is equal to 30 mg of plain phentermine. In many of the patients, it is initiated at a dose of a dose of 37.5 mg in its salt form, but in some of the patients, the dose can be reduced to half based on the patient's conditions, and this drug should be preferably given before breakfast. Particularly one or two hours before breakfast, this drug should be taken.