Phentermine, like many other prescription drugs, works with neurotransmitters in the brain. It is a centrally-acting stimulant and is a constitutional isomer (not to be confused with stereoisomer) of methamphetamine. It stimulates neuron bundles to release a particular group of neurotransmitters known as catecholamines; these include dopamine, epinephrine (also known as adrenaline), and norepinephrine (noradrenaline). The anorectic activity seen with these compounds is thus likely due to their effect on the central nervous system, which is consistent with current knowledge about the central nervous system and feeding behavior. This is the same mechanism of action as other stimulant appetite suppressants such as diethylpropion and phendimetrazine. The neurotransmitters signal a fight-or-flight response in the body which, in turn, puts a halt to the hunger signal. As a result, it causes a loss in appetite because the brain does not receive the hunger message.
Contraindications and warnings
Patients with the following should not use Phentermine:
- An allergy to any ingredient in Phentermine or other sympathomimetics (eg, pseudoephedrine)
- Are also taking dexfenfluramine, fenfluramine, furazolidone, guanadrel, guanethidine, or have taken a monoamine oxidase inhibitor (MAOI) (eg, phenelzine) in the last 14 days
- Have severe high blood pressure, an overactive thyroid, glaucoma, heart or blood vessel disease, or severe narrowing of the blood vessels
- Are in an agitated state, or have a history of substance abuse
Some medical conditions may interact with Phentermine, patients with the following should consult with their doctor before using phentermine:
- Are pregnant, planning to become pregnant, or are breastfeeding
- Are taking any prescription or nonprescription medicine, herbal preparation, or dietary supplement
- Have allergies to medicines, foods, or other substances
- Have a brain or spinal cord disorder, hardening of the arteries, high blood pressure, diabetes, or high cholesterol or lipid levels
Some medicines may interact with phentermine, such as the following:
- Dexfenfluramine, fenfluramine, furazolidone, or MAOIs (eg, phenelzine) because the risk of serious side effects, such as increasing headache, high blood pressure, slow heart rate, elevated temperature, or possibly fatal lung problems, may be increased
- Serotonin specific reuptake inhibitors (citalopram (Celexa), escitalopram (Lexapro), fluoxetine (Prozac, Sarafem), fluvoxamine (Luvox), paroxetine (Paxil), or sertraline (Zoloft)) because the risk of their side effects may be increased by Phentermine
- Guanadrel(Hylorel) or guanethidine(Ismelin) because their effectiveness may be decreased by phentermine
- Antacids: Antacids may decrease the excretion of phentermine.
- Carbonic anhydrase inhibitors (acetazolamide, dichlorphenamide, methazolamide): Carbonic anhydrase inhibitors may decrease the excretion of phentermine.
Generally, phentermine appears to be relatively well tolerated. It can produce side effects consistent with its catecholamine-releasing properties, e.g., tachycardia (increased heart rate) and elevated blood pressure, but the incidence and magnitude of these appear to be less than with the amphetamines. Because phentermine acts through sympathomimetic pathways, the drug may increase blood pressure and heart rate. It may also cause palpitations, restlessness, and insomnia. Additionally, phentermine has the potential to cause physical and psychological dependence.
More common symptoms include:
- Increased blood pressure
- Sense of well-being
- Dry mouth
- Unpleasant taste
- Blurred vision
- Heartburn/Acid reflux
- Change in sexual desire
- Irregular heartbeat
- Nausea or vomiting
- Skin rash or itching
- Stomach pain