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Phentermine is a stimulant medication used as an appetite suppressant to assist with weight loss in combination with a reduced calorie diet and exercise program. The medication is approved by the FDA as a short-term adjunct therapy for adults with certain comorbid conditions who are overweight or obese. Phentermine works on the brain to reduce hunger and on the cells to increase metabolism to burn more fat. The common side effects of phentermine include dry mouth and a restless feeling, and the medication is contraindicated in individuals with cardiac disease, cerebrovascular disease, or who are pregnant, planning to become pregnant, or breastfeeding.


Duration of Use

When the FDA approved phentermine in 1959, it was approved for short-term use, typically interpreted as about three months. However, obesity is a long-term disease, and newer weight loss medications, such as the combination of phentermine and topiramate, have been approved for longer-term use. While most weight loss physicians use phentermine on a longer-term basis off-label, it is important to note that the benefit of losing weight should be considered higher than the risks associated with the medication.


Combination with Other Weight Loss Medications

Phentermine is increasingly being used as a component of combination weight loss medications, such as Qsymia, and is often used with other weight loss medications, including bupropion, metformin, diethylpropion, and topiramate. In the opinion of many weight loss physicians familiar with phentermine, it is a relatively well-tolerated medication that works best in combination with a reduced calorie diet and exercise under careful medical supervision.


Phentermine is available as a 15 mg, 30 mg, or 37.5 mg capsule or tablet, and the strength of the medication that is most effective depends on an individual's situation and the weight loss physician's recommendation. While there are variations in the formulation in terms of extended-release or immediate-release, the capsule or tablet should be the same.


Individuals should tell their doctor and pharmacist if they are allergic to phentermine or any other medications, or if they are taking any other medications. Phentermine works best in combination with a diet program, and individuals should follow the diet and exercise program that their doctor has given them. If a dose of phentermine is missed, individuals should take the missed dose as soon as they remember it, but if it is almost time for the next dose, they should skip the missed dose and continue their regular dosing schedule.


Phentermine is an amphetamine and is the most widely used anti-obesity drug in the U.S. While phentermine is the agent of choice among physicians specializing in obesity treatment, the use of this drug for obesity treatment by other physicians has long been curtailed due to misconceptions regarding phentermine safety. Concerns of phentermine-induced adverse cardiovascular reactions and phentermine-induced addiction have had a profound negative impact on phentermine prescribing. However, after 52 years of use, there is no evidence in the peer-reviewed medical literature to support the hypothesis that phentermine has significant human addiction potential.

Side Effects

Common side effects of phentermine include dry mouth, unpleasant taste, diarrhea, constipation, and vomiting. Some side effects can be serious, such as increased blood pressure, heart palpitations, restlessness, dizziness, tremor, insomnia, shortness of breath, chest pain, swelling of the legs and ankles, and difficulty doing exercise that an individual has been able to do. Individuals should call their weight loss or primary doctor if they have any unusual problems during their treatment with phentermine.

Trade Names

Phentermine is marketed under various trade names, including Adipex P (immediate release) and Ionamin.

W8MD Doctors in Philadelphia

W8MD doctors in Philadelphia prescribe phentermine and Adipex for weight loss. Individuals can contact W8MD for more information about insurance coverage, the cost of phentermine, and other cheap weight loss prescription options in Philadelphia.


Phentermine may interact with medications such as dexfenfluramine, fenfluramine, furazolidone, or MAOIs (e.g., phenelzine), which are contraindicated due to the risk of serious side effects, such as increasing headache, high blood pressure, slow heart rate, elevated temperature, or possibly fatal lung problems. Guanadrel (Hylorel) or guanethidine (Ismelin) effectiveness may be decreased by phentermine. Antacids may decrease the excretion of phentermine, and carbonic anhydrase inhibitors (acetazolamide, dichlorphenamide, methazolamide) may also decrease the excretion of phentermine.

Frequently asked questions

  • What to avoid while taking phentermine? While taking phentermine, it is important to avoid alcohol, as it can increase the risk of side effects such as dizziness, confusion, and impaired judgment. It is also important to avoid taking other medications that may interact with phentermine, such as other weight loss medications, MAO inhibitors, and certain antidepressants.
  • What should I expect on the first day of phentermine? On the first day of taking phentermine, you may experience side effects such as dry mouth, constipation, and a decrease in appetite. You may also experience an increase in energy and a sense of euphoria, as phentermine is a stimulant.
  • How soon will I start losing weight on phentermine? The amount of weight you lose on phentermine will vary depending on your individual circumstances, such as your starting weight and lifestyle habits. However, many people start to see weight loss within the first few weeks of taking phentermine.
  • How do I get the best results from phentermine? To get the best results from phentermine, it is important to follow a healthy diet and exercise plan. You should also take phentermine as directed by your healthcare provider and avoid alcohol and other medications that may interact with it.
  • Can you drink caffeine while taking phentermine? While caffeine is not contraindicated with phentermine, it may increase the risk of side effects such as jitteriness and insomnia. Therefore, it is best to limit your caffeine intake while taking phentermine.
  • Should you drink caffeine while taking phentermine? It is not necessary to drink caffeine while taking phentermine, and it may be best to limit your intake in order to avoid potential side effects.
  • How soon before bed should I take phentermine? Phentermine is a stimulant and can interfere with sleep, so it is best to take it earlier in the day, ideally in the morning or early afternoon.
  • What is the peak time of phentermine? The peak time of phentermine varies depending on the individual and the formulation of the medication. However, most forms of phentermine have a peak effect within 3-4 hours of taking it.
  • Does phentermine make you poop more? While constipation is a common side effect of phentermine, it does not typically cause an increase in bowel movements.
  • How can I speed up weight loss with phentermine? To speed up weight loss with phentermine, it is important to follow a healthy diet and exercise plan. You should also stay hydrated, get enough sleep, and avoid alcohol and other medications that may interfere with phentermine.
  • How much weight can you lose a week with phentermine? The amount of weight you can lose on phentermine varies depending on your individual circumstances. However, many people lose an average of 1-2 pounds per week.
  • What are the best foods to eat on phentermine? The best foods to eat on phentermine are those that are high in protein and fiber, as these nutrients can help you feel full and satisfied. Examples include lean meats, fish, eggs, vegetables, and fruits.
  • Why am I still hungry on phentermine? While phentermine can decrease your appetite, it may not completely eliminate it. It is still important to make healthy food choices and eat regular meals in order to fuel your body and avoid feeling hungry.
  • Can phentermine make you skinny? Phentermine can help you lose weight, but it is not a magic pill that will make you skinny. It is important to follow a healthy diet and exercise plan in order to achieve and maintain weight loss while taking phentermine.
  • Does phentermine give you energy? Yes, phentermine is a stimulant that can increase energy levels and reduce fatigue. This is one of the reasons it is sometimes used to treat conditions such as narcolepsy and attention-deficit/hyperactivity disorder (ADHD).
  • What are the results of phentermine after 3 months? The results of phentermine after 3 months can vary depending on your individual circumstances, but many people experience significant weight loss during this time period. It is important to continue following a healthy diet and exercise plan in order to maintain weight loss results. Your healthcare provider can give you more specific information on what to expect based on your individual situation.

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W8MD Weight Loss, Sleep & Medspa Centers is a network of medical centers located in New York, Pennsylvania, New Jersey and surrounding areas that provide comprehensive care for weight loss, sleep disorders, and aesthetic treatments.

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