The Keto Diet and Phentermine

keto diet phentermine

The ketogenic diet, more commonly referred to as the “keto diet”, is one of the most popular diets for weight loss right now. This very low carb eating plan promises rapid and dramatic weight loss to its followers. However, medical experts remain dubious about its safety and efficacy. So are the keto diet and phentermine weight loss pills a good combination? Here we’ll discuss the keto diet: its origins, pros & cons, and the advisability of combining phentermine and low carb or fad diets.

REMINDER: We’re not doctors and no part of this post in any way constitutes medical advice, nor an endorsement of using the keto diet and phentermine together. This post is purely informational. Always speak with a licensed medical professional before undertaking any major dietary change.

What is the keto diet?

The popular keto diet is a very low carbohydrate, moderate protein, high fat diet. There are several variations of this latest fad diet, but the “standard” keto diet calls for a distribution of 75% fat, 20% protein and 5% carbs. This means that just 5% of daily calories come from carbs – that’s just 75 calories (about 19g of carbs) for a person eating 1500 calories per day!

There also exist several other variations of the keto diet including the: high-protein ketogenic diet (60% fat, 35% protein, 5% carbs), cyclical ketogenic diet (5 days on, 2 days off) and  targeted ketogenic diet (eat more carbs around workouts). The last two are primarily designed for athletes have not been extensively studied. [1]

However, not everyone needs to restrict carbs quite this much. Many people can achieve a state of ketosis by eating anything less than about 50 grams of net carbs (total grams of carbs – total grams of fiber) per day. Other low-carb diets that are still colloquially referred to as “keto” allow for still higher levels of daily carbohydrate.

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Where Keto Came From

The ketogenic diet was originally developed as a treatment for refractory pediatric epilepsy, or seizure disorders in children that aren’t successfully controlled by medications. Since this diet forces the brain to shift towards using ketone bodies (from fat) instead of glucose (from carbs) as its primary energy source, adhering to a keto diet can help reduce seizure frequency and severity. The strictest clinical patients follow a 4:1 keto diet wherein they get 80% of calories from fat (ideally MCTs, which are more ketogenic) and only 20% of calories from protein & carbohydrates. The clinical goal of this diet is to provide just enough protein and calories to support sufficient growth and weight status, while absolutely limiting carbohydrate intake to reduce epileptic symptoms.

Medically, epilepsy management remains the only widely-accepted use of the ketogenic diet. Scientists are researching the diet’s effectiveness in treating or managing other neurological disorders and cancer, but so far there’s no conclusive evidence to support the implementation of this diet to treat or manage these conditions. [2]

How the Keto Diet Works

The basic idea behind the keto diet is to drop your intake of carbohydrates so low that your body and brain switch to ketone bodies as their main source of fuel.

Fueling Your Body on a Balanced Diet

Under normal conditions, the body relies on glucose as its primary source of energy. Glucose, or sugar, is the most biologically-available source of fuel and it’s our brain’s favorite food. When we’re eating a normal, balanced diet about 50-60% of our calories come from carbs. Grains, fruits, vegetables (especially starchy vegetables like potatoes, corn and peas) and dairy products contribute to your daily carbohydrate intake. Sugar – whether it’s white sugar, brown sugar, honey, maple syrup or agave – is also digested as a simple carbohydrate. When we ingest carbs, we use insulin to break down sugars in our foods and convert it them into glucose that our cells can use for energy. This is how someone eating a balanced diet gets most of their energy, and the quick turnaround of glucose metabolism is why it’s normal to feel more energized soon after eating and then start feeling hungry a few hours later.

Fueling Your Body on Keto

However, when the body doesn’t receive enough calories or carbs (or can’t use them, in the case of uncontrolled diabetes), the liver starts synthesizing ketone bodies to fuel the body and brain. These molecules, which are derived from fatty acids, serve as an efficient fuel substitute for the missing glucose. The high-density nature of ketone energy may explain (at least partially) why a true keto diet proves neuroprotective against seizures and other neurological conditions. [3] Ketone tests and keto strips determine if a person is in ketosis by measuring the levels of ketone bodies in their blood, breath or urine.

keto diet and phentermine_egg

Potential Benefits of Keto

Proponents of the keto diet note many possible advantages of following this restrictive regimen. The most-touted benefits are rapid & dramatic weight loss and lower insulin levels over the short-term.

Adopting a ketogenic diet allows followers to lose weight quickly and without counting calories. Many people report they feel less hungry while in ketosis, especially after the initial transition period. More, studies show that adults lose more weight following a high-fat/low-carb diet instead of a low-fat diet. [4] [5] However, experts note that much of this initial weight loss is simply water weight due to the drop in glycogen.

A very small study (of 12 people) also indicated that the ketogenic diet could reduce insulin sensitivity. This effect would reduce risk for developing type 2 diabetes and contribute to the management of several other conditions like metabolic syndrome and PCOS. [6]

Still, many health professionals warn that the risks of this extreme diet outweigh any potential benefit.

Possible Risks of Keto

The long-term side effects of using ketosis for weight loss have not been studied, nor has the combination of the keto diet and phentermine weight loss pills. There’s not enough evidence to determine if this diet is safe for everyone, and health experts warn that the restrictive plan can do more harm than good (especially when it’s not medically-supervised).

Common side effects of the keto diet include constipation and dehydration (two problems already experienced by many phentermine users), as well as electrolyte and micronutrient deficiencies. This diet may also increase the risk of kidney stones (especially worrisome for Qsymia users), gallbladder problems, bone fractures and/or menstrual irregularities. [3]

Long-term studies of children who use follow keto diets for seizure control also exhibit higher total cholesterol and LDL (bad cholesterol) levels. While keto proponents claim this diet improves heart health, long-term research indicates that it worsens patients’ lipid profiles.

One user and multiple registered dietitians also have also warned that entering ketosis weakens muscle development, including in the heart. The heart is the most important muscle in the body and weakening it could prove extra-problematic for phentermine patients. [7]

If you are taking Qsymia, do NOT start a low-carb or ketogenic diet. The topiramate portion of this medication is an anti-seizure medication that should not be combined with any sort of high-fat, low-carb diet like Atkins or keto. [8]

Keto Diet and Phentermine

While many people adopt the keto diet to lose weight quickly while taking phentermine, this fad diet is NOT a good choice. Medical experts warn that this diet does more harm than good, plus its potential interaction with phentermine remains unknown.

Problem 1: Keto isn’t proven (or even tested) for use alongside phentermine

Evidence does not yet support the keto diet as an alternative or addition to pharmaceutical therapy for weight loss. The long-term effects of ketosis for weight loss have not yet been studied and there is not sufficient evidence to suggest it is safe for everyone. In fact, some evidence indicates that it could prove dangerous. [7] Never follow keto, Atkins, or any other low-carb/high-fat diet while taking Qsymia.

Problem 2: Keto isn’t a sustainable dietary pattern

Moreover, research repeatedly shows that the most effective diets for weight loss are the ones we can stick to long-term. Very few people maintain a super-restrictive eating plan like keto for years at a time. It is much more commonly used as a temporary diet that people stop following as soon as they reach their goal weight. Unfortunately, the problem with radical fad diets like keto is that, in most cases, the weight starts to come back as soon we start eating normally again. This is especially true with keto given that much of the initial weight loss can be attributed to a loss of water weight.

Problem 3: Keto undermines your opportunity to develop healthy habits while taking phentermine

Finally, using the keto diet and phentermine together takes away a major advantage of your prescription. Phentermine is a powerful drug to help jumpstart your weight loss, but the idea is that this prescription medication acts as an addition to overarching lifestyle changes. It’s important to use your three months on phentermine to establish healthier eating and exercise habits for life. It shouldn’t be a “one and done” twelve week commitment that you abandon once you reach your goal weight. Since the keto diet isn’t a sustainable eating plan, this need for a long-term weight loss mindset is another reason that it’s not a good choice to do the keto diet and phentermine together. Instead, practice consistent dietary balance and portion control to set yourself up for long-term weight loss success!


If you still want to eat low-carb or keto while taking phentermine, discuss it with your doctor and a registered dietitian first. Severely limiting or eliminating entire macro groups (in this case carbohydrates) is hazardous even for the healthiest people, and the risk increases when you have other conditions. This potential danger is compounded by combining a strong prescription medication like phentermine (or worse-yet Qsymia) with the restrictive diet. As a result, it’s critical that you speak with your doctor before making any radical dietary changes.


Do you use the keto diet and phentermine together? Share your questions and comments with us in the comments section below!



[1] R. Mawer, “The Ketogenic Diet 101: A Detailed Beginner’s Guide,” 17 Jun 2017. [Online]. Available:

[2] Wikipedia, “Ketogenic diet,” 11 May 2018. [Online]. Available:

[3] S. Fan, “The fat-fueled brain: unnatural or advantageous?,” 1 Oct 2013. [Online]. Available:

[4] B. Brehm, R. Seeley, S. Daniels and D. D’Alessio, “A randomized trial comparing a very low carbohydrate diet and a calorie-restricted low fat diet on body weight and cardiovascular risk factors in healthy women.,” Journal of Clinical Endocrinology & Metabolism, pp. 1617-23, 2003.

[5] N. Bueno, I. de Melo, S. de Olivera and T. da Rocha Ataide, “Very-low-carbohydrate ketogenic diet v. low-fat diet for long-term weight loss: a meta-analysis of randomised controlled trials.,” The British Journal of Nutrition, pp. 1178-87, 2013.

[6] G. Boden, K. Sargard, C. Homko, M. Mozzoli and T. Stein, “Effect of a low-carbohydrate diet on appetite, blood glucose levels, and insulin resistance in obese patients with type 2 diabetes.,” Annals of Internal Medicine, pp. 403-11, 2005.

[7] L. Seegert, “The Keto Diet Is Gaining Popularity, but Is It Safe?,” 7 Jun 2016. [Online]. Available:

[8] US National Library of Medicine, “Topiramate,” 15 Dec 2017. [Online]. Available:

13 thoughts on “The Keto Diet and Phentermine”

  1. Avatar

    I personally take Phentermine and am currently on a full ketogenic diet. I’ve been in ketosis for about a month and a half, and I’ve lost 20 lbs (From 205 to 185) . The weight loss is definitely due to the diet in my opinion, but the ability to maintain the diet is probably due to the medicine. It’s working for me, and I think I’ll be able to keep the weight off afterwards.

    1. Avatar

      Hi Jacob, thanks so much for your comment! Congratulations on your weight loss success 🙂 That’s great to hear that the Keto diet is working so well for you!! We always love to hear about people’s personal experiences with phentermine. Keep up the good work!

  2. Avatar

    I have been doing Keto and phentermine since February and have lost over 60lbs. I think your article is completely false in terms of claiming the limitations of Keto. The common misconception is that that carbs are good for you when in all actually The amount of carbs that you can eat in the ketogenic diet is conducive to including fruits and vegetables into your diet daily. What it does not include is all of the processed carbs, starches, grains that everyone likes to eat. I eat less than 24g of carbs a day, have amazing mental clarity and energy, And when I go on vacation I increase my carbohydrates, eating all of those processed foods that we love on vacation, and when I start feeling like trash at the end of my vacation, I am happy to go back to my low carbohydrates healthy food. I’m not self sabotaging anymore, so I think this article is in accurate in stating that it’s not sustainable for a long period of time.

    1. Avatar

      Big congratulations on your weight loss success, Heather!! We love hearing diverse opinions on these topics – thanks for so much for sharing your experience 🙂

    2. Avatar

      Hi Heather! I am looking for information on the subject because I’m just starting on phentermine but really looking forward to maximize the three month usage window. I’ve been reading about the Keto diet and I think I can sustain it, since I love carbs but I really don’t like processed foods, my carbs intake comes from fancy home made desserts, pasta and such (my husband is an awesome cook), but it really can be controlled by talking to him and spacing the frecuency of those foods. I’m starting to exercise (the main issue with my overweight is an absolute sedentarism, as I work from home and don’t move from the screen for several hours) and I would love if you could share a bit more about your experience, specially about the % of fruits and vegetables that have worked for you. Thank you and have a great day!

      1. Avatar

        I did take the pills for three months ten years ago and lost sixty pounds. I loved the energy it gave me but I could not sleep at all!! Then when I stopped the meds, I gained it all back and much more and I have been struggling since then till I started keto three months ago. If you are doing keto right, you do not need phentermine because you will not be hungry! I fast 16 hours now and only eat two meals. I lost 32 pounds in three months. Yes, it’s less than the diet pills but I am eating 1800 calories a day now whereas with the pills I could only eat 1100, which is not healthy at all. The good thing about keto is that I don’t have anymore pain in my body and I stopped all pain meds. My brain fog is gone and I have so much energy and can exercise for hours!! For me keto is not s diet, it’s a lifestyle that has changed my life in a positive way!!

        1. Avatar

          Congrats on your weight loss, Helen! Thanks so much for sharing your experience 🙂

  3. Avatar

    No disrespect, but I totally disagree with this article, I been on keto going on 3months and I’ve lost 41 pounds and I know that’s not all water weight! The water weight comes off mainly the first couple of weeks and that’s for any type of diets! It seems that you really don’t know about dieting at all less long keto. When in ketosis your way focus, clear headed and full of freaking energy. You at as if those crappy carbs. You talking bout really good for you anyway, their the main reason why people are looking for ways to lose weight in the first place I been doing keto off and on for 20 plus years I know what it does it a beast! For anyone looking to try it out, I strongly suggest it! Always get with medical physician first

    1. Avatar

      Hi Bella! The standard ketogenic (keto) diet is 75% fat, 20% protein and 5% carbs. Still, someone following a strict keto diet won’t consume very many (if any) starchy foods, which may be why popular perception is that the keto diet is a “no carb” diet.

      Best regards,

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