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Phentermine is a drug primarily used as an appetite suppressant.

Although relatively well-known, few people are aware of the fact that it has the basic chemical structure of an amphetamine, which means that it acts as a stimulant, causing an elevation of blood pressure and faster heart rates. Phentermine is in a class of drugs called anorectics which decrease appetite by changing brain levels of certain neurotransmitters. 
 
Phentermine is available in 30 mg and 37.5 mg tablets. It is a short-term-use drug normally as part of hasa diet and behavior-modification plan to treat obesity. Although often abused in other purposes, it is typically prescribed for individuals who are at increased medical risk because of their weight, as opposed to cosmetic weight loss. 
Because of that, it has been removed from the market by the FDA. 
 
Phentermine is still available by itself in most countries, including the U.S. but the problem is that, because it is an amphetamine, individuals may develop an addiction to it. It is classified as a controlled substance in many countries and is internationally considered a schedule IV drug under the Convention on Psychotropic Substances.   
 

Phentermine is also known under the following brand names:

  • Adipex-P®
  • Anoxine-AM®
  • Fastin®          
  • Ionamin®
  • Obephen®
  • Obermine®    
  • Obestin-30®
  • Phentrol®

History

This drug first received an approval from the FDA as an appetite-suppressing drug back in 1959. It was previously sold as Fastin® from King Pharmaceuticals for SmithKline but later, in 1998, it was removed from the market.
 
Since 1959, there have been almost no clinical studies performed. The most recent study was in 1990 which combined phentermine with fenfluramine or dexfenfluramine, and became known as Fen-Phen. Although this combination has never been approved by the FDA, this study has proven that Fen-Phen was more effective than diet and exercise, with few side effects. However, future studies proved that nearly 30% of people taking fenfluramine of dexfenfluramine had abnormal valve findings and that’s one of the reasons why it has been taken off the market by the FDA.

Mechanism of action

As previously mentioned, this drug is chemically almost identical to amphetamine, and, like many other prescription drugs, works with neurotransmitters in the brain. It is a centrally-acting stimulant, which means that phentermine stimulates neuron bundles to release a particular group of neurotransmitters known as catecholamines. This special kind of neurotransmitters includes dopamine, epinephrine (also known as adrenalin), and norepinephrine (noradrenaline), and has anorectic properties.
 
Anorectic activity is present most likely due to the effect on the central nervous system, because it is basically the same mechanism of action that other stimulant appetite suppressants such as diethylpropion and phendimetrazine have. All this is based on the regulation of the “fight-or-flight response”. (Neurotransmitters activate a fight-or-flight response in the body which, in turn, puts a halt to the hunger signal. The result is logical – it causes a loss in appetite because the brain does not receive the hunger message.)

FDA recommendations

Presently, it is recommended by the Food and Drug Administration that phentermine should be only used short-term. This period is usually interpreted as 'up to 12 weeks'. fter that, one should follow up with ordinary non-pharmacological approaches to weight loss, such as healthy dieting and exercise. 
The usage of this medication is so limited for drug tolerance reasons, to prevent the body from adjusting to the drug completely. This is somewhat controversial because many clinical studies have been done on this mater, and they showed that phentermine did not lose effectiveness in a 36-week trial. 
A second reason is an increased risk of insomnia. To avoid this problem, it is generally recommended that the drug be taken either before breakfast or 1-2 hours after breakfast.

How to Take this Medicine?

Phentermine should be taken on an empty stomach, once per day, 30-60 minutes before breakfast. Because it is rather big, the tablet may be broken or cut in half; it should never be crushed or chewed. As already explained, the medication should be avoided in the evenings because it may cause sleeplessness.  

Side effects

Generally, phentermine appears to be relatively well tolerated. Like all medications, it may cause side effects because of catecholamine-releasing properties. 
 
Most common side effects that could be caused by these substances are:
 
  • tachycardia
  • increased heart rate
  • increased alertness
  • increased blood pressure
  • increased heart rate
  • palpitations
  • restlessness
  • insomnia
  • euphoria
  • psychological

Addiction

Phentermine is habit-forming. A patient can become physically and psychologically dependent on this medication, and withdrawal effects may occur if stopped suddenly after several weeks of continuous use. That’s why all patients should practice coming off this medication gradually.

Contraindications for Phentermine

Allergy: A patient should report to a doctor any previous unusual or allergic reaction to this medicine, as well as amphetamine, dextroamphetamine, ephedrine, epinephrine, isoproterenol, metaproterenol, methamphetamine, norepinephrine, phenylephrine, phenylpropanolamine, pseudoephedrine, terbutaline, or other appetite suppressants.  
Diet: During the usage of this medication, a patient should follow a reduced-calorie diet in order to lose weight. Even after the patient has stopped using it, the diet and exercise regimen should be continued in order to maintain the new weight.
Pregnancy: If a pregnant woman takes this medicine in high doses, or more often than the doctor has directed, it may cause withdrawal symptoms in the newborn baby.  
Breast-feeding: Medications such as Diethylpropion and benzphetamine pass into breast milk but; it is not known if other sympathomimetic appetite suppressants pass into breast milk. However, use of sympathomimetic appetite suppressants during breast-feeding is not recommended.
Children: The use of these medicinations by children younger than 16 years of age is not recommended.
 
Other contraindications:
  • Cardiovascular disease
  • Atherosclerosis
  • Glaucoma
  • Agitation
  • High blood pressure
  • Hyperthyroidism
  • History of drug abuse

Drug Interaction

Phentermine should not be taken by patients with glaucoma, hyperthyroidism, or a history of drug abuse or psychotic illnesses, to avoid detrimental interaction with medications used for these conditions. Phentermine is not recommended for patients with poorly controlled high blood pressure. Insulin requirements may change in patients on phentermine, which means that diabetes could also be contraindication. As with any other medications, alcohol can cause a drug interaction.  
 
Although certain medicines should not be used together at all, in other cases two different medicines may be used together even though an interaction might occur.

When talking about appetite suppressants, it is important to mention that there are several possible contraindications with other medications such as:

  • Amantadine
  • Amphetamines or
  • Caffeine  
  • Chlophedianol  
  • Cocaine or
  • Medicine for asthma or other breathing problems
  • Medicine for colds, sinus problems, or hay fever or other allergies  
  • Methylphenidate  
  • Nabilone  
  • Pemoline  
  • Appetite suppressants (diet pills)
  • Selective serotonin reuptake inhibitors (citalopram [e.g., Celexa®], fluoxetine [e.g., Prozac®], fluvoxamine [e.g., Luvox®], paroxetine [e.g., Paxil®], sertraline [e.g., Zoloft®])
  • Monoamine oxidase (MAO) inhibitors (furazolidone [e.g., Furoxone®], isocarboxazid [e.g., Marplan®], phenelzine [e.g., Nardil®], procarbazine [e.g., Matulane®], selegiline [e.g., Eldepryl®], tranylcypromine [e.g., Parnate®])
  • Tricyclic antidepressants (amitriptyline [e.g., Elavil®], amoxapine [e.g., Asendin®], clomipramine [e.g., Anafranil®], desipramine [e.g., Pertofrane®], doxepin [e.g., Sinequan®], imipramine [e.g., Tofranil®], nortriptyline [e.g., Aventyl®], protriptyline [e.g., Vivactil®], trimipramine [e.g., Surmontil®])

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