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When Should I Take My Phentermine?

when to take phentermine

In order to maximize the weight loss potential of your phentermine prescription, you should always follow the instructions given to you by your phentermine doctor. Still, sometimes it can be confusing if things weren’t made clear when you picked up your prescription and you’re left thinking, “When should I take my phentermine?”. Here, we’ll explain everything you need to know in this step-by-step guide about how and when to take your phentermine, as well as what to avoid while taking this medication.

Before You Take Phentermine

It is important to clarify that phentermine is not suitable for everyone.

First and foremost, you must not take phentermine if you are pregnant, as it can harm an unborn baby. You should also not take it if you are breastfeeding, as it can be passed into breast milk and may harm a nursing baby. You should not take phentermine if you have a history of heart disease, severe or uncontrolled blood pressure, overactive thyroid, glaucoma, a history of pulmonary hypertension, or if you have a history of drug or alcohol abuse. You should also not take phentermine if you have taken an MAO inhibitor (e.g. isocarboxazid, linezolid or tranylcypromine) in the past 14 days, given that a dangerous drug interaction could occur. Lastly, you should not take phentermine if you are allergic to it, or if you have had an allergic reaction to other diet pills, amphetamines, stimulants or cold medications.

Once you arrive at the appointment to be evaluated for phentermine, tell your doctor if you have any of the following: high blood pressure, diabetes, kidney disease, a thyroid disorder, or an aspirin allergy. It is vital to disclose all of your medical history; even if you think it is unrelated. It is important for your safety that your doctor is properly informed of any potential there may be for a negative reaction to the medication.

When to Take Phentermine

If your doctor decides that you are suitable for a phentermine prescription, he or she should provide you with clear instructions for when to take it. In general, phentermine should be taken at the beginning of the day and normally on an empty stomach. Some brands of phentermine should be taken between half an hour and an hour before breakfast, or while others are taken within two hours of eating your first meal. These instructions will vary with different phentermine pills.

If you’re taking a smaller dose (or splitting tablets), you will may take your phentermine up to three times per day. Just remember: never exceed the maximum dosage of 37.5mg per day. Given the many possible variations in when and how to take phentermine, always take phentermine exactly as prescribed by your doctor and follow all directions listed on your prescription label.

Phentermine is a stimulant and can cause insomnia, so you shouldn’t take this medication later than 6pm. If you take phentermine late at night, you could experience problems sleeping. That being said, if you have a different waking schedule (such as if you work nights or shifts), you may need to adjust when you take phentermine. You should explain any relevant schedule concerns to your doctor a the initial appointment so that he or she can devise a suitable phentermine schedule for you.

If you miss a dose, you should take the dose as soon as you remember. However, if it is almost time for your next dose, then it is better to skip the missed dose. You should NEVER take extra medicine to make up for the missed dose.

woman taking phentermine

How to Take Phentermine

Phentermine can be prescribed in tablet, capsule or ODT (orally disintegrating/dissolving tablet) form. Available doses range from 8mg to 37.5mg, taken 1-3 times per day. To give you an idea of the possibilities: Adipex-P is a 37.5mg tablet or capsule taken once per day, while Lomaira is an 8mg tablet taken up to three times per day, and Suprenza is an ODT taken once per day.

Your individual phentermine dosage will be determined by your doctor, according to your specific needs and requirements. Phentermine should never be taken in larger or smaller amounts, more or less often, or for longer periods than prescribed. Always consult your doctor if you think that the medication isn’t working properly and want to adjust your dose, as taking more than your recommended dosage of phentermine can cause serious, life-threatening side effects.

Common phentermine side effects include dry mouth, changes in taste, hyperactivity or restlessness, insomnia, diarrhea, constipation, headaches, dizziness and nausea, amongst others. However, if you feel that the dosage is too strong for you or if you experience any symptoms of severe side effects, contact your doctor immediately.

You should also not stop taking phentermine suddenly, or you could experience unpleasant withdrawal symptoms. Ask your doctor how to safely stop using this medicine if you do decide you want to stop taking it, before the end of your prescription.

What to Avoid on Phentermine

Drinking alcohol can increase the incidence or severity of certain side effects, so you should avoid alcohol while taking phentermine. Phentermine may also impair your thinking or reactions, so be careful if you drive, operate heavy machinery or do anything which requires you to be alert. During the first few days of phentermine it’s advisable to monitor how you feel in order to self-assess how the medication is affecting your body. If you feel uncertain about taking on certain tasks, seek advice from your doctor.

In your first consultation you should tell your doctor about any medications or supplements that you are currently taking, even if they are non-prescription. Many of medications or supplements interact with the phentermine and could cause the medication to be less effective, or even prove dangerous. Your doctor may recommend that you take additional vitamins or supplements with phentermine, but if not you should check with your doctor before you decide to take any additional products yourself. You should also tell your doctor about any medications you start or stop using during your treatment with phentermine, especially antidepressants.

In order to make sure your phentermine prescription is as effective as possible, it is also a good idea to cut down on foods and drinks which cause additional acidity in your urinary pH. Experiments have shown that, when compared to an alkaline pH, an acidic urinary pH causes phentermine to be excreted twice as fast and therefore, less effective. Unhealthy acidifying foods and drinks to avoid include alcohol, coffee, soda, butter, and ice cream.

The main issue to stress here, is that you should always follow your doctor’s instructions. Phentermine is a prescribed medication and should be treated accordingly. It is also important that you are completely honest with your doctor in your consultations, so that he or she is well-informed about your medical history and can treat you effectively. If you have any concerns or questions about phentermine, it is always a good idea to ask your doctor – both for you to be better informed and so that your doctor can improve their care for you and future patients.

We hope that you now feel confident about phentermine and how to take it, but if you have any feedback about this article, please get in touch with us by commenting below!

Editor’s Note: This post was originally published January 2014, but was expanded and updated for accuracy in April 2018. 

67 thoughts on “When Should I Take My Phentermine?”

  1. Avatar

    my dr prescribed phyermine to me in what seems like a high dose twice a day he also is currently my psycheds prescriber which include anti depressants, I tried taking them last week early during the day and was still up anxious at night all night! I’m on 37.5 twice a day and am confused on when I’m to eat especially the second dose when I’m not hungry at all and food isn’t an option. I recently took a medication that put a ton of weight on me in a short amount of time after the weight gain I got discouraged and ate every little Debbie in my state and probably yours. I have a leg injury so exercise isn’t an option I see that dr in two weeks after waiting months. I have other medical issues going on and my dr is informed of all of this and everything I didn’t post

    1. Sally

      Hi Lana,
      Generally phentermine is taken every day but if your doctor thinks it’s ok and it works for you to just take it during the week then there’s no reason why you shouldn’t take it this way.
      Sally, phentermine.com

    1. Sally

      Hey Amber,
      We have an article about taking phentermine if you have thyroid problems which you might find interesting: Phentermine and Thyroid. Lots of people take phentermine alongside thyroid medications but it’s always best to check with your doctor before combining medications. If you have different doctors for your thyroid and to prescribe the phentermine then they should both be made aware that you are taking these two medications.
      Sally, phentermine.com

      1. Avatar

        Hi Sally, the article you referenced is helpful and everything my dr explained but what I really want to know is how far apart should I take the phentremine from my thyroid replacement? I take it around 5am on an empty stomach. Can they be taken together or wait an hour or several in between? Thanks!

        1. Avatar

          Hi Amy! The doctor that prescribed you both medications (or perhaps a knowledgeable pharmacist) would probably be the best person to talk to about specific timing of the two medications.

  2. Avatar

    Hi! I just stsrted phentermine on the 14th of August. My first and second dose gave me a lot of energy. Now I am back to no energy and feeling blah. I have lost 14 lbs thou. I see my doctor next week. Should I see about uping my dosage or taking it twice a day?

    1. Sally

      Hi Sharon,
      If you’re taking the maximum 37.5mg then it’s very unlikely your doctor would increase your dosage but it is worth mentioning your decrease in energy to see if he or she can alter your prescription or give you any advice on ways to increase your energy naturally or through supplements such as B12 shots.
      Sally, phentermine.com

  3. Avatar

    Hi, I work midnights (11-7) and wanted to start this medicine, what is the best time to take it and what is a good eating schedule?

    1. Sally

      Hi Michele,
      We published an article which explains how to manage your lifestyle with phentermine which you might find useful: https://www.phentermine.com/blog/manage-lifestyle-phentermine/ In your case, as you work nights it’s best to take phentermine when you wake up, rather than at any particular time of day. You should then have your first meal about an hour or so later, which could be a breakfast-type meal if you want, or could be dinner if it’s easier to eat the same as whoever you live with. Then it would be best to have your ‘lunch’ during your shift, and then a light meal towards the end of your shift, around 4 hours or more before you go to bed.
      Hope that helps!
      Sally, phentermine.com

    1. Sally

      Hi Denise,
      Normally your doctor will advise when to take your phentermine, but if you’re not sure then the usual recommendation is to take phentermine on an empty stomach first thing when you wake up, and then eat around an hour later. It’s best to check with your doctor for confirmation if you can too.
      Sally, phentermine.com

  4. Avatar

    I have just been giving some of this medication to take and I have not took it in years . Do you recommended taking 1/2 of it first couple days?

    1. Sally

      Hi Erica,
      Yes, that is usually a good idea as it is an aggressive drug so people find that taking just half to start with minimizes the side effects. If you find that you’re ok then you can increase it to a whole one after a few days.
      Sally, phentermine.com

  5. Avatar

    Hello I am starting phentramine and need ideas on foods exercise and the best time to take it what has worked for you let me know please

    1. Sally

      Hi Veronica,
      Thanks for your message! There are tons of articles on this blog with info on all these subjects – just type something you’d like to know about in the search bar and you’ll see all the related articles. To connect with other phentermine users for advice and to share experiences, remember to join our Facebook support group!
      Sally, phentermine.com

    1. Sally

      Hi Sylvia,
      It should be fine but it’s best to ask your doctor about combining anything with phentermine – he or she will be in a better position to know your medical history and other factors which would indicate whether it is safe or not.
      Sally, phentermine.com

  6. Avatar

    I started taking phentermine 8 days ago. My dose is 37.5 and it is prescribed to be taken once a day. I’ve been taking it first thing in the morning and then had breakfast 15-20 minutes later. In my first week I lost 1lb (yeah, o-n-e). I read that it can also be taken 1-2 hrs after breakfast – will this change the dynamics of weight loss?

    1. Sally

      Hi Helen,
      It’s best to take your phentermine when you are told to take it, but this is almost always before eating and on an empty stomach. It should also be taken as early as possible in the day so as not to interfere with sleep, which is very important for weight loss. Lastly, a 1lb loss is still good, especially if you are working out like you should be, as this can increase muscle tone and make it look like you’ve lost less weight when in fact you’ve shed a few pounds of fat and built up your lean muscle – the overall loss doesn’t always show the full story. Keep at it!!
      Sally, phentermine.com

    1. Sally

      Hi Chris,
      We would advise that you ask your doctor to see if there is the possibility of an interaction between phentermine and medical marijuana. Since both are prescription only then you would be able to ask when being prescribed.
      Sally, phentermine.com

  7. Avatar

    Hi, my ANRP has prescribed the 37.5 pill 1/2 pill twice a day. I take it 1/2 hr before I eat breakfast and lunch (my pharmacist suggestion). Will I have successfull weight loss with this dosage? Any suggestions?

    1. Sally

      Hi Judy,
      Yes, these are fine if you want to relieve a headache whilst taking phentermine.
      Sally, phentermine.com

  8. Avatar

    Ive been reading up on these pills I recently gained 30 pounds with a new bc and wanted to try these how can I get them prescribed?

  9. Avatar

    I am starting this tomorrow but was told by my doctor to take 1/2 hour after breakfast and 1/2 hour after lunch. I keep reading everyone is taking this before these meals. Which is right?

    1. Sally

      Hi Brenda,
      Usually phentermine is taken before meals but sometimes doctors advise taking it after, which may be related to your personal medical history. It is best to follow what your doctor has told you but if you can it could be useful to check why your were advised to take the phentermine after meals so you know what to watch out for when it comes to side effects and reactions.
      Hope that helps!
      Sally, phentermine.com

      1. Avatar

        Thank you. He actually said to take 1/2 pill an hour after eating breakfast then again after lunch. After a week switch to one full pill an hour after breakfast. I will check with him and find out why 🙂 thanks again

  10. Avatar

    How do you actually take the pill? My doctor didn’t say anything about the method of taking it. I’ve read on some sites that you should allow it to dissolve on the tongue but your site doesn’t say anything about that. Which way is correct? Again, my doctor didn’t give me any guidance so I’ve just been swallowing it whole.

    1. Sally

      Hi Jen,
      The only phentermine brand which dissolves on the tongue is Suprenza, which you can read about on our site here: https://www.phentermine.com/suprenza/. If you have normal phentermine pills then swallowing them whole is the most effective way to take them so you are taking them correctly.
      Sally, phentermine.com

    1. Sally

      Hi Danielle,
      We have an article on insomnia which has some useful advice for you: https://www.phentermine.com/phentermine-side-effects/insomnia/ Many people who experience stronger side effects when they first start taking phentermine find it helpful to split their dosage in half if it’s in pill form. Taking a lower dosage helps users to get used to the effects of phentermine and then when they feel ready they can start to take the full dosage and should not experience such severe side effects as before. You should also make sure to take it as early as possible in the morning so that it has a chance to wear off before you go to bed.
      Sally, phentermine.com

  11. Avatar

    Should I take excedrin to help with these mild headaches. I’m afraid the caffeine wont mix right with the Phentermine 37.5 but it’s all I have on hand aside from Aleve PM which I wouldn’t want to take during the day.

    1. Sally

      Hi Ashley,
      A small amount of caffeine is fine in conjunction with phentermine, so it depends how much caffeine the Excedrin contains. Anything up to around 100mg of caffeine up to twice a day is fine. When you get the chance to go to a pharmacy we recommend ibuprofen as this reduces swelling and acts quickly. However, this will only treat the pain not the cause, so be sure to drink plenty of water throughout the day to avoid getting more headaches on phentermine.
      Sally, phentermine.com

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