Could I Suffer From Headaches If I Am On Phentermine?

by Ahmed Zayed, MD on May 13, 2024
Last updated on May 13, 2024

Headaches, those annoying throbs that hit us out of nowhere, have tons of triggers. Perhaps you skimped on sleep or didn’t drink enough water. At times, it’s the blazing sun overhead or a bug that’s got you feeling off. And you know what else? Certain pills turn out to be major headache culprits, totally raining on your parade. Curious about whether taking phentermine could lead to a headache? You’re in the right spot to find out. Jump in to discover some neat facts about this headache puzzle. Your noggin will be super grateful you stayed to dig deeper.

man suffers headache sitting on a sofa

What is phentermine?

Phentermine is a prescription medication used in conjunction with reduced-calorie diet, regular exercise, and other lifestyle modifications to help men and women lose weight. Sold under brand names such as Adipex P, Suprenza and Lomaira, phentermine was first developed in 1952 and it is one of the oldest pharmacologic agents approved by the FDA.

Intended for short-term use only, the drug is prescribed only to men and women whose BMI is 30 or higher, but it is also prescribed to patients whose BMI is 27 or higher in the presence of health problems contributed by obesity such as cardiovascular disease risk factors and diabetes.

While there are tons of different weight loss drugs available with doctor’s prescription phentermine is one of the most popular agents of them all. Its ever-growing use is not due to popularity only, but the fact that the growing body of evidence confirms its efficacy. For example, Kim et al. found that short-term administration of phentermine has the tremendous potential to promote significant weight reduction and the decrease of waist circumference (1).

The drug is a stimulant, and it works to suppress a patient’s appetite in order to lower their calorie intake and prevent overeating. Phentermine increases the amount of energy the body uses, and it could also affect certain parts of the brain to promote weight loss.
The popular weight loss medication belongs to the group of drugs called sympathomimetic amines. That being said, specific underlying mechanisms the drug uses to help users slim down still need to be fully elucidated.

It’s important to mention the drug is approved for 12 weeks use only, and the greatest deal of weight loss occurs in the first few weeks. This is not a type of agent that people can or should take on a long-term basis. Besides Adipex-X and other drugs, phentermine is also available in combination with other medications such as topiramate. These combination drugs are also considered effective in helping people lose weight. One study investigated the impact of phentermine and topiramate combination on weight loss and found that it is a well-tolerated treatment for obesity and weight-related metabolic complications (2).

What are the side effects of phentermine?

All medications, prescription or not, come with a certain risk of side effects including phentermine. Despite the fact, this is a highly effective drug for weight loss it still has the potential to induce some adverse reactions (3), but it doesn’t mean that every patient is bound to experience them.

Below, you can see the list of the most common adverse reactions of phentermine (4):

  • A headache
  • Bad taste in the mouth
  • Vomiting
  • Constipation or diarrhea
  • Dry mouth

Besides the above-mentioned most prevalent side effects of phentermine, other reactions may also occur. They include (5):

  • Pain in chest, arm, or jaw
  • Shortness of breath
  • Dizziness
  • Skin rash, itching, redness
  • Problems walking or weakness
  • Trembling or shaking
  • Hypertension (high blood pressure)
  • Fast heartbeat
  • Swelling in legs and feet
  • Upset stomach
  • Sleep disturbances
  • Unusual feelings or behaviors such as sensitivity, jitteriness

One can decrease the risk of side effects by adhering to the dosage prescribed by their doctor. Avoid increasing or decreasing the dose on your own, and it’s also important to make healthy lifestyle modifications.

Why does phentermine cause a headache?

As seen above, headache is one of the most common side effects of phentermine. Now you’re probably wondering about the reasons why the drug can induce a headache. At this point, there are no studies that focused on the underlying mechanisms through which phentermine causes headaches. It’s important to conduct research on this subject so that both doctors and patients know more about headaches due to phentermine, why they happen, and what to do about them.

Stimulant effects

Phentermine is a chemical that affects every person in a different manner. That could explain why some patients experience no headache whatsoever. One theory that could explain the reason why phentermine gives you headaches is its stimulant nature. Stimulants lead to vasoconstriction, i.e., they narrow blood vessels. As a result, the heartbeat becomes fast, but you also experience uncomfortable, pulsating headache (6).


Another reason why more than half of patients on phentermine get headaches is dehydration. When prescribing phentermine, your doctor will probably tell you to drink plenty of water while taking this drug. You should take this advice seriously due to the fact that staying hydrated curbs cravings, protects your kidneys, and it also nourishes and improves the quality of your skin.

Now let’s go back to the above-mentioned side effects; you can notice that some patients make experience skin problems. Dehydration can induce skin-related issues just as it can contribute to a headache. Since phentermine can lead to dehydration, it can subsequently cause a headache too. After all, headache is one of the biggest telltale signs of dehydration. So, why does lack of fluid in the body lead to a headache?

While this subject also requires more research a commonly accepted explanation is that dehydration can, just like stimulant effects, lead to vasoconstriction, i.e., narrowing of blood vessels. They narrow in order to try and regulate levels of fluid in the body. But, as a result, it becomes more difficult for blood along with nutrients and oxygen to reach the brain which leads to a headache (7).

Skipping meals, i.e., appetite suppression

The mechanism of action that phentermine utilizes to boost weight loss but can also give you a headache. How? Well, as you’re already aware phentermine works to suppress appetite. You don’t eat as much as you used to. Appetite suppression may make you want to skip meals. When paired with calorie-restrictive diet, this drug can cause low blood sugar, increased stress or both. What happens next is that you deal with uncomfortable, pulsating headaches that annoy you.

Sleep deprivation

Phentermine is a stimulant which means it keeps your brain alert, aware, and awake. That explains why many patients on phentermine report they don’t get much sleep. However, we need a good night’s rest to function properly. When you sleep, the brain uses that opportunity to refuel and recharge. But, if you’re not getting enough shut-eye, the brain doesn’t rest either, and headache occurs. In this case, a person experiences a tension-type of a headache which prevents them from doing their daily tasks and affects their quality of life in the long run.

Will I get a headache if I am on phentermine?

Okay so half of the patients who take phentermine complain they get a headache (8), but does it mean you’ll also have this problem? Not really! What makes one person at a higher risk of getting phentermine-caused headache than the other? Risk factors include:

  • Failing to adhere to doctor’s instructions – doctor advice isn’t a formality, but a serious recommendation that you need to follow. Failing to adhere to these instructions increases your risk of side effects, including a headache.
  • First-time phentermine use – a risk of adverse reactions including headache increases among new users of phentermine. In other words, if you’re using the drug for the first time, the risk of side effects is higher compared to someone who has already used the medication. Your body is just getting used to the drug, so it’s expected that some reactions may occur.
  • Drinking and smoking – let’s face it; both drinking and smoking are unhealthy habits we need to ditch in order to prevent various health problems and improve the management of conditions we may have. Drinking and smoking are bad news for almost any other kind of drug and phentermine is not the exception. For instance, smoking also constricts your blood vessels so if you combine it with phentermine which can do the same, and it becomes crystal clear why smokers deal with a headache when taking the drug.

What to do if you have a headache on phentermine?

Now that you know that phentermine has the potential to induce a headache and reasons why it happens, it’s also important to address solutions to this problem. There is no law which says there is nothing you can do about it. Below, you can see different things that can help you manage and prevent a headache when taking phentermine:

  • Consult your doctor – if you’re concerned about headaches, it’s useful to consult your doctor who will recommend a proper way of resolving them. This is particularly important if a headache is severe
  • Identify triggers and avoid them – headache usually has a trigger, and a great way to prevent or manage it is to identify that trigger. Some people forget to drink enough water, and others skip their meals, then there’s also someone who drinks too much coffee, and doesn’t sleep enough. As seen above, phentermine can lead to a headache due to all these effects, but you need to identify them first. Take some time to think about whether you eat your meals regularly or whether you drink enough water etc. Once you identify the trigger, you can create a plan to avoid it and prevent or manage your headache. For example, you can set up a reminder to drink a glass of water
  • Take a painkiller – an easy way to manage a headache is to take a painkiller, but make sure it is over-the-counter. Avoid getting some stronger drugs unless your doctor says it’s okay
  • Stick to diet modifications – our body needs certain nutrients to remain healthy and to lose weight in a safe and natural manner. If you don’t consume a well-balanced nutrient-rich diet, you may not achieve desired results. Make sure your diet is rich in fruits, vegetables, fiber, antioxidants, minerals, vitamins, and other healthy nutrients
  • Use essential oils – to promote relaxation and calmness you may want to try essential oils such as lavender and peppermint. Massage them on your temples, inhale, or soak a small towel or a washcloth in a bowl of water with a few drops of essential oil and place it on your forehead
  • Other things to do: meditate, practice yoga, try acupuncture, try to get enough sleep, avoid strong smells, drink green tea


A headache is one of the most common side effects of phentermine. Various causes are involved including sleep deprivation, skipping meals, stimulant effects, and others. Adhere to doctor’s instructions, consult your healthcare provider if necessary, and be proactive in implementing some healthy changes to your lifestyle to avoid or manage a headache caused by phentermine.


PhentermineDoctors has strict sourcing guidelines and relies on peer-reviewed studies, academic research institutions, and medical associations. You can learn more about how we ensure our content is accurate and current by reading our editorial policy.

Leave a Reply

Your email address will not be published. Required fields are marked *