Phentermine is a prescription medication used for the short-term treatment of obesity in people with additional risk factors, such as diabetes or high cholesterol.
It should always be combined with a low-calorie diet, regular physical activity, and long-term lifestyle changes.
Phentermine is only available by prescription and is sold under brand names like Adipex-P, Lomaira, and Qsymia (when combined with topiramate). It comes in capsules, tablets, extended-release tablets, and orally disintegrating tablets. The dosage and treatment plan must be determined by a doctor, ideally with guidance from a nutritionist.
What phentermine is used for
Phentermine is prescribed for short-term weight loss in people with:
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Obesity, with a body mass index (BMI) greater than 30 kg/m²
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Overweight, with a BMI over 27 kg/m², along with risk factors such as diabetes, high cholesterol, sleep apnea, or osteoarthritis
Phentermine should always be paired with a balanced, low-calorie diet, regular exercise, and healthier eating habits.
How phentermine works
Phentermine belongs to a class of medications called anorectics. It acts on the central nervous system by stimulating the release of neurotransmitters such as norepinephrine and dopamine, which help reduce appetite.
In addition, its effects on weight loss are linked to an increase in metabolic rate, which helps the body burn more energy.
How to take phentermine
Phentermine is available in different forms, with adult dosages and administration as follows:
1. Capsules
Capsules may contain 5 mg, 10 mg, 15 mg, or 30 mg of phentermine. The usual recommended daily dose ranges from 15 mg to 30 mg.
2. Tablets
Tablets may contain 6.4 mg, 15 mg, or 30 mg of phentermine. Depending on the doctor’s instructions, they can be taken one to three times a day, up to a maximum of 30 mg daily.
3. Extended-release tablets
Extended-release tablets contain 30 mg of phentermine and are generally taken once daily, as prescribed by a doctor.
Capsules and tablets should be swallowed with a glass of water 30 to 60 minutes before meals.
4. Orally disintegrating tablets
Orally disintegrating tablets may contain 15 mg, 30 mg, or 37.5 mg of phentermine. The recommended dosage must be prescribed by a doctor, but in most cases, one tablet is taken in the morning.
These tablets can be taken with or without food. They should be handled with dry hands and placed directly on the tongue to dissolve.
Phentermine should not be taken after 7:00 p.m. to avoid insomnia. The lowest effective dose should always be used, based on the doctor’s recommendation, to minimize side effects.
How long can you take phentermine?
Phentermine is typically prescribed for up to 4 weeks, or intermittently (4 weeks on treatment followed by 4 weeks off).
Side effects
Possible side effects of phentermine include:
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Insomnia
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Palpitations and rapid heartbeat
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Nervousness or euphoria
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High blood pressure
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Ischemic events
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Dry mouth or unpleasant taste
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Diarrhea, constipation, nausea, or vomiting
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Hives
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Changes in libido or impotence
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Excessive sweating
Because long-term use can cause dependence and withdrawal symptoms, the dose should always be reduced gradually under medical supervision.
Can phentermine cause death?
Phentermine can be dangerous if taken in higher amounts than prescribed. Overdose may lead to chronic intoxication, with symptoms such as severe insomnia, irritability, hyperactivity, skin problems, personality changes, psychosis, vomiting, diarrhea, or abdominal cramps.
Excessive doses of phentermine can cause fatal poisoning, coma, or death.
If overdose is suspected, emergency medical care is required.
Contraindications
Phentermine should not be used by:
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Children under 12 years old
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Older adults
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People who consume alcohol
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Pregnant or breastfeeding women (phentermine crosses the placenta and is found in breast milk)
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People with anorexia, insomnia, weakness, psychotic disorders, a history of drug abuse, or alcohol misuse
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People with moderate to severe high blood pressure or cardiovascular disease, including arrhythmias
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People with glaucoma or hyperthyroidism
In people with diabetes, insulin or oral diabetes medications may need to be adjusted when combined with phentermine.
Phentermine must not be taken with monoamine oxidase inhibitors (MAOIs), as this can trigger a hypertensive crisis.
Other drugs that can interact dangerously with phentermine include:
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Duloxetine
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Pseudoephedrine or phenylephrine
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Procarbazine
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Sibutramine (withdrawn from the US market)
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SSRIs, such as citalopram, escitalopram, fluoxetine, fluvoxamine, paroxetine, and sertraline
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Stimulants, such as dexmethylphenidate, methylphenidate, or modafinil
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Venlafaxine
You are advised to share your full health history with your doctor so that he or she can determine whether this medication is right for you.