Keto diet steak

The Keto Diet and 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 its followers rapid and dramatic weight loss. However, medical experts remain dubious about its safety and efficacy. 

So are the keto diet and phentermine 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.

Note: The text below contains affiliate links – we may earn a small commission if you click on them. This post is for informational purposes only. We are not doctors, and this article in no way constitutes medical advice nor an endorsement of keto and phentermine. 

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 daily!

Other variations of the keto diet include 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 and have not been extensively studied (1).

However, not everyone needs to restrict carbs 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, still colloquially referred to as “keto,” allow for higher levels of daily carbohydrates.

Want to know exactly what to eat so you can stay in range and lose weight with keto? Take this quiz to learn your ideal macro ranges, and then check out your CUSTOM meal plan – complete with downloadable grocery lists and keto-friendly restaurant recommendations.

Origins of Keto

The ketogenic diet was initially developed as a treatment for refractory pediatric epilepsy: seizure disorders in children that aren’t successfully controlled by medications.

Since this diet forces the brain to use 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.

However, scientists are researching the diet’s potential effectiveness in treating or managing other neurological disorders and cancer (2).

Want to know how many carbs you can eat and still lose weight? Take this free quiz to learn your ideal macro ranges (protein, fat & carbohydrate) for weight loss with keto.

How It 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 primary 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 fuel source and is our brain’s favorite food.

When we’re eating a regular, balanced diet, about 50-60% of our calories come from carbs.

Grains, fruits, starchy vegetables, 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 them into glucose that our cells can use for energy.

This is how someone eating a balanced diet gets most of their energy.

The quick turnaround of glucose metabolism is why it’s normal to feel energized right after eating and then begin to feel sluggish a few hours after you eat.

Fueling Your Body on Keto

However, when the body doesn’t receive enough calories or carbs (or can’t use them because of a condition like diabetes), the liver starts making ketone bodies to fuel the body and brain.

These molecules, which form from fatty acids, are an efficient fuel substitute for the missing glucose (3).

Traditionally, 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.

Benefits of Keto

Benefits of Keto

Proponents of the keto diet note many possible advantages of following this diet.

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.

Moreover, recent studies show that adults lose more weight following a high-fat/low-carb diet instead of a low-fat diet (4,5).

Experts, however, note that much of this initial weight loss is simply water weight due to the drop in glycogen.

One small study (of 12 people) also indicated that the ketogenic diet could reduce insulin sensitivity.

This effect would reduce the risk of developing type 2 diabetes and contribute to managing several other conditions like metabolic syndrome and PCOS (6).

Eight weeks of meals planned JUST FOR YOU! Enter your preferences and receive a two-month meal plan with downloadable grocery lists and keto-friendly restaurant recs. Get started today!

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.

Health experts warn that the restrictive plan may harm some people more than good.

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.

A low-carb, high-fat diet may also increase the risk of kidney stones (especially worrisome for Qsymia users), gallbladder problems, bone fractures, and/or menstrual irregularities (3).

Studies show that long-term children who follow keto diets (for seizure control) also have higher total cholesterol and LDL (bad cholesterol) levels.

While keto proponents claim this diet improves heart health, long-term research indicates that it actually worsens patients’ lipid profiles.

Ketosis can also impede muscle development, including in the heart.

The heart is the most important muscle in the body, and you don’t want to weaken this vital organ, especially while taking a stimulant like phentermine (7).

Do NOT start a low-carb or ketogenic diet if you are taking Qsymia (phentermine HCL + topiramate-ER). Topiramate should not be combined with any sort of high-fat, low-carb diet like Atkins or keto (8).


Keto Diet and Phentermine

Many patients, and even doctors, see keto as an effective weight loss strategy during phentermine treatment.

This trendy diet has helped millions of people lose weight and aligns with popular advice to eat fewer carbs and less sugar.

However, very low carb/high-fat diets may not be safe for everyone, and there is no research to establish the advisability of combining phentermine and keto.

Always speak with your doctor before undertaking any significant dietary or lifestyle changes, including combining the keto diet and phentermine.

Here are three reasons NOT to combine the keto diet and phentermine:

1. The interaction between phentermine and keto remains unknown

Evidence does not yet support the keto diet as an alternative or addition to phentermine. Moreover, researchers do not know about the safety or long-term effects of using ketosis for weight loss.

NEVER follow keto, Atkins, or any other low-carb/high-fat diet while taking Qsymia.

2.  Strict keto isn’t a sustainable dietary pattern for most people

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, and, in most cases, the weight returns as soon you start eating normally again.

3. Keto undermines the development of healthy habits

Phentermine is a potent appetite suppressant, but its ultimate purpose is to help you establish long-term lifestyle changes.

As a result, it’s essential to use your three months on phentermine to establish healthier eating and exercise habits that you can maintain long-term.

If you still want to eat low-carb or follow keto while taking phentermine, discuss it with your doctor and dietitian first. They can help you develop an individualized, healthy plan for weight loss.

Go into your appointment prepared with an individualized plan for weight loss with keto! Click here to see your easy, 7-day plan.

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.

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  1. Ketogenic and low carb diets have been used for long periods of time by many people over several decades without any issues. In fact this is how many families were eating in the 70s before the food pyramid gained popularity promoting 4 servings of fruit and 6 to 11 servings of grains a day, which was preposterous, but people started following it. That was never healthy. Eating sugar every day was never healthy. A low carb diet was actually pretty normal and wasn’t difficult to follow at all for most people. We also didn’t eat all day long, which started as a fad in the 80s to “increase metabolism” when it actually does just the opposite and snack food manufacturers filled the grocery stores with easy to eat, terrible quality foods to keep the habit going. Keto is not a fad or dangerous, or difficult. It’s normal to use your stores of glucose up every day. The question is whether or not phentermine is effective or useful at all with a diet that doesn’t promote overeating? If it is being used to suppress appetite and control overeating “behavior” when appetite isn’t being overstimulated in the first place, it seems it might not be very useful.

  2. I have been following a very clean version of the diet and I’ve lost 30 lbs so far with LUPUS and Hashimoto’s.

    Starting Blood Work Compared to Halfway Point
    Cholesterol 284 to 267
    VLDL from 56 to 20!
    Triglycerides 281 to 99
    Hashimoto’s Antibodies are lowest I’ve ever seen
    LUPUS feels like I may be in remission

    All other numbers are also going down. LDL did stay but I’ve adjusted my diet a bit to help with that by increasing the right kinds of fats and not eating as much red meat.

    IT’S WHAT YOU EAT NOT JUST CARBS OR NO CARBS. The keto diet makes your body burn fat. Have you done a study to find out who lost more fat or muscle with phentermine on different VERY CLEAN diets?

    I am eating absolutely no processed foods, no sugars with the exception of berries and some other fruits, and avoiding fake sugars. Breakfast is usually simple with a cup of green tea and I might have an organic free range egg then some supplements like VD3, K2, B12. A typical lunch would be another egg, about 2 oz of fish or chicken, half an avocado, a salad (possibly with spinach) with 2 tablespoons olive oil, a Brazil nut (for Hashimoto’s), and some green tea with possibly some collagen peptides. Dinner might be some organic chicken wing recipe and another quick veggie. It’s about 1500 calories or so and high in protein. I’m a 5’4” tall 60 yr old woman so this can be done. You can eat better and drop the weight.

    The reason I needed phentermine was due to the hypothyroidism along with my age. I needed something to help me stay on track while I changed my routine, eating habits, and included cardio and weights. I don’t plan on living my life as a slave to autoimmune disorders.

    So unless you all know exactly what people in some study or trail on a ‘keto diet’ ate, you don’t know what you’re talking about. I tried weight watchers and you know what happened? I sat in their 30 minute commercials, oh I mean ‘meetings’, and ended up hooked on their shakes and bars. I hardly ate any real food for weeks. My husband finally told me it has to stop. I wasn’t even cooking dinner anymore. I don’t care if it’s Weight Watchers or Keto or the Mediterranean Diet, you have to eat clean real foods or it’s a junk diet and you will see all your numbers and your health go down.

  3. I started Keto a month prior to beginning Phentermine. I had done Keto previously. And I stay GF so the weight stays off longer once I finish Keto.

    I lost 10 pounds in 3 weeks just on Keto. And then I added Phentermine and lost an additional 6 pounds in 2 weeks on that. I keep the sodium and electrolytes up. As well as drinking Lemon water or Dr.Berrys (Keto-Aid) he’s got a video on YouTube about it. It’s supper helpful. Makes me feel great.

    I’ll have to remember to come back to this page. In a few months to discuss how it was for the 3 month period. 😅

  4. I mostly eat a keto diet and try to stay away from carbs. I am not perfect with this, but keto and phentermine seemed to be my magic bullet… until I started getting kidney pain. I have not recovered yet and it’s been over a
    month since I stopped the pills. I have always been very healthy. I really want to go back on the pills, does anyone have any sound, or even medical, expertise here to give me feedback? Thank you.

  5. I am taking phenterine to help control my appetite in conjunction with intermittent fasting 20:4 and keto. 2.5 weeks down and I’ve lost 13 lbs so far. I feel great and my too hungry during my fast periods. I take my phenterine first thing in the morning on an empty stomach. I am sleeping fine at night and have tons of energy during the day.

  6. My question isn’t specially about the keto diet. I’ve taken phentermine in the past and had amazing results. Currently, I’ve been drinking Pruvit ketones natural, sometimes with caffeine and some days without. I’m not eating a ketogenic diet though. I wanted to know if I could continue drinking the ketones while taking phentermine? I’m interested in taking phentermine again.

    1. Hi Chris,

      Please check with your doctor before taking any other medication or stimulants alongside phentermine.

      Best regards,


  7. I’ve been on the keto diet for 6 weeks and haven’t lost anything. I’m a diabetic taking medications. I can’t get my ketones over 0.2 , the information I have read to lose weight reported that I need to maintain at 1.6. From a medical standpoint that is dangerous for a diabetic like myself. On the other hand it has dropped my A1C. I’m getting 15-20 carbs, 60-80 g Protein, and 60-100 in fats per day. I like to lose 50-60 pounds, Help please! Thanks

    1. Hi Kathy,

      We would recommend for you to speak to a nutritionist. You need to follow a diet that fits your individual needs. You could also consider taking a natural supplement.

      We hope that helps! We wish you the best of luck moving forward in your weight loss journey.


  8. Is it ok to take phentermine 37.5 and the Keto BHB. Not the keto diet but just this pill? I have controlled high blood pressure but it’s been ok with the phentermine

    1. Hi Julie,

      🙂 Always check with your doctor and pharmacist about drug combinations. It is normally recommended to AVOID taking multiple weight loss medications at the same time.


  9. I’ve been doing keto with intermittent fasting (2 meals a day, fasting 16 hours a day) for 5 weeks.
    I started using 30mg of Phentermine a day 1 week ago to see if it helps.
    I’ve lost 12.5 kg so far (started at 124kg), as well as 4 inches from my waist.
    I feel as though the Phentermine hasn’t affected my ketosis, but it has helped reduce cravings and helps with an energy boost that encourages me to exercise more.
    Thankfully I’m active enough that I still feel tired at the end of each day and can still sleep.

  10. I heard that a keto diet calls for a distribution of 80% fat, 20% protein, and no carbs. Am I right?

    1. 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,

  11. 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

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

      2. I started working from home and immediately began putting weight on! I invested in a desk that I can sit or stand to work. A little pricey but better for overall well being! I also removed my candy dish and only gave a tumbler of ice water to drink! (I do use those individual flavorings tho).
        My friend bought one of those things that you can pedal while sitting down and feels it’s helping her! Anyway, best of success to you! We are all in this thing together!

  13. 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. 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!

    1. Hi Michelle, thank you so much for letting us know!! We’ve fixed it now 🙂

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