Why Does Phentermine Make Me Sleepy & Tired? (EXPLAINED)

January 10, 2024 |

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Do you feel tired and sleepy after taking Phentermine?

This is a commonly reported side effect of the medication and one that many people find concerning. In this blog post, I will discuss the causes of phentermine-related fatigue and sleepiness, as well as some tips for how to combat these side effects.

Keep reading to learn more.

I’ll also share with you one really safe and better Phentermine replacement I discovered when researching for this piece. It does everything Phentermine does and even more.

Key Highlights:
  • Phentermine may cause fatigue and difficulty in sleeping in some people due to its stimulant effects, increased metabolism, and potential for anxiety and low blood pressure.
  • Other common side effects of Phentermine include dizziness, headache, nausea, and constipation.
  • Serious side effects of Phentermine include insomnia, hypertension, and primary pulmonary hypertension.
  • An alternative to Phentermine is PhenQ, a natural appetite suppressant that contains no side effects and is available over the counter without a prescription.
  • Click here to learn more about PhenQ for free

Before we delve into the question of whether Phentermine causes you to sleep and or feel tired – let us remind ourselves what Phentermine is and how it works in the human body.

What is Phentermine?

Phentermine is a potent weight-loss medication classified as an amphetamine. It operates by effectively suppressing the appetite, making it a valuable tool for individuals aiming to shed excess pounds.

The core mechanism of Phentermine is its ability to stimulate the central nervous system, triggering the release of norepinephrine.

This neurotransmitter is responsible for vital physiological responses, such as an increased heart rate, elevated blood pressure, and heightened breathing rate. These effects combine to reduce the sensation of hunger and diminish food cravings, ultimately facilitating weight loss.

Furthermore, Phentermine goes beyond appetite suppression; it revs up the body’s metabolism.

This means that your body burns calories more rapidly, contributing to more efficient weight loss. This metabolic boost is a result of the heightened central nervous system activity initiated by Phentermine.

While Phentermine can be a powerful tool for those seeking to lose weight, it’s essential to emphasize that it is intended for short-term use. Typically prescribed for a few weeks or months, prolonged use is discouraged because the body can develop a tolerance to the drug, diminishing its efficacy and potentially leading to side effects.

Health professionals recommend its use under their supervision and guidance.

For fitness enthusiasts and athletes aiming to achieve rapid weight loss and appetite control, Phentermine can be a strategic short-term solution.

It may prove invaluable when a quick reduction in weight or body fat percentage is necessary for competitions or events. However, athletes should be vigilant regarding any regulations or prohibitions related to the use of such medications within their sports organizations.

How Does Phentermine Work?

The hypothalamus – a small, almond-shaped structure in the brain – is responsible for controlling appetite, body temperature, water balance, and other important functions.

When you take Phentermine, it sends a signal to the hypothalamus telling it to release more norepinephrine. This hormone then tells the body to break down stored fat and use it for energy.

This means that more of the hormone is available to activate the central nervous system.

Phentermine also binds to receptors in the brain that are responsible for controlling appetite. When these receptors are activated, they send a signal to the body that tells it to eat less.

So, Phentermine not only activates the central nervous system but also blocks the receptors that control appetite. This combination of effects can help you lose weight quickly.

Now that we know a little more about what Phentermine is and how it works, lets discuss whether it causes fatigue or sleepiness as a side effect.

sleeping woman

Why Does Phentermine Make You Feel Tired?

Yes, Phentermine will make you feel tired because it is a stimulant. It works by increasing your heart rate and breathing rate, which can make you feel exhausted. It also increases the amount of norepinephrine and dopamine in the brain, which are two chemicals that help to stimulate the central nervous system. When these chemicals are increased, they can make you feel more alert and awake.

There are other several reasons why Phentermine might cause you to feel tired.

  • Metabolic Fluctuations:

When you’re on a Phentermine regimen, you may encounter fluctuations in your metabolism. This can lead to changes in your energy levels. As your body adapts to the medication and undergoes metabolic adjustments, you might experience periods of tiredness.

  • Energy Peaks and Valleys:

Phentermine, as a stimulant, can initially provide bursts of energy. However, as the stimulating effects wear off, you might experience a “crash,” which can manifest as fatigue. This is a common pattern seen with some stimulant medications.

  • Disrupted Sleep Patterns:

Phentermine can sometimes lead to changes in your sleep patterns. Sleep disturbances, including insomnia or difficulty falling asleep, can result in daytime fatigue. This is an indirect effect related to altered sleep.

  • Individual Variability:

It’s important to note that individual responses to medications can vary. While Phentermine is generally considered a stimulant, some individuals may have idiosyncratic reactions, leading to feelings of tiredness. Your body’s unique chemistry plays a role in how you respond to the medication.

Does Phentermine Cause You to Feel Sleepy?

Phentermine is an appetite suppressant, not a sleep aid. So it should NOT cause you to feel sleepy. In fact, from my research, I found out that it may cause difficulty in sleeping in some people. While some people experience drowsiness, others feel more alert. I believe that it depends on the person taking it and their body’s reaction to the drug.

If you are feeling sleepy after taking Phentermine, speak with your doctor about it as this may be a sign of another health condition.

Otherwise, try to drink plenty of water, eat healthy foods, and get regular exercise to help you stay alert and awake.

Again, Phentermine can cause you to feel tired for the following reasons:

  • It speeds up your metabolism, which can make you feel tired
  • It can cause you to feel anxious or nervous
  • It can cause you to have lower-than-normal blood pressure
  • It reduces energy levels

Before I delve into one of my recommended alternatives to Phentermine (discovered recently) and how they work in suppressing your appetite and losing weight – let’s quickly cover some other side effects.

What To Do & The Precautions to take When tired on Phentermine

Tip #1. Stay Hydrated

Tiredness can be a common side effect of Phentermine, and it may be exacerbated by dehydration. To combat this, it is highly recommended to drink more water throughout the day. Staying well-hydrated can help alleviate fatigue and ensure that your body functions optimally while on the medication.

Tip #2. Timing of Doses

To minimize the impact of tiredness and to avoid potential sleep disturbances, it’s advisable to schedule your Phentermine doses strategically.

Generally, you should take the last dose of the day approximately 4-6 hours before your intended bedtime. This allows the medication’s stimulating effects to wear off as you approach sleep, increasing the likelihood of restful and uninterrupted sleep.

Tip #3. Consult Your Doctor

If despite proper timing and hydration, you’re still experiencing difficulty falling asleep or staying asleep, it’s important to consult your healthcare provider. They can provide guidance on potential adjustments to your medication regimen or recommend sleep aids that are safe to use in combination with Phentermine.

Never self-medicate with over-the-counter sleep aids without your doctor’s approval, as they may interact with the medication or exacerbate side effects.

Tip #4. Adherence to Dosage Instructions

It’s crucial to strictly adhere to the dosage instructions given by your healthcare provider. Phentermine is a prescription medication, and deviating from the recommended dosage can have adverse effects, including increased tiredness.


Do not alter your Phentermine dosage without consulting your doctor first, as they can provide guidance on any necessary adjustments.

Tip #5. Avoid Abrupt Discontinuation

When you’ve completed your prescribed course of Phentermine or if your doctor advises you to discontinue the medication, it’s important not to stop suddenly. Abruptly discontinuing Phentermine can lead to withdrawal symptoms, including depression and severe tiredness. Your doctor will guide you through a safe and gradual tapering-off process to minimize these withdrawal effects.


Remember that Phentermine is a powerful medication that should be used under the close supervision of a healthcare professional. It’s crucial to communicate openly with your doctor about any side effects you experience, such as tiredness, as they can help you manage these effects effectively and safely.

Other Side effects of Phentermine

Here are common effects:

  • Dizziness
  • Headache
  • Nausea
  • Constipation
  • Diarrhea
  • Bad taste in your mouth
  • Constipation

These effects are mild and last for a while. But it should be monitored carefully if it worsens over time.

On the other hand, there are serious side effects as well – they include:

  • Insomnia (trouble sleeping)
  • Erectile dysfunction in men
  • Hypertension (high blood pressure)
  • Primary pulmonary hypertension (PPH)
  • Restlessness
  • Tremors
  • Severe headaches

The above effects are life-threatening and always warrant a visit to your doctor.

Are There Any Alternatives to Taking Phentermine?

You may already know this – that Phentermine has been a Schedule IV controlled substance for a while now. And, as such, it’s not available over the counter like some of its alternatives are.

There are a few that I’ve found and will be covering shortly – but first, let me ask you this:

What if there was an alternative to taking Phentermine that had ZERO side effects, uncontrolled, and available over the counter, online, and worldwide?

Would that be something you’re interested in?

Well, a few weeks ago while I was conducting research online I stumbled on a brand called PhenQ.

After a bit more research, I learned that PhenQ does exactly what Phentermine does but without ANY of the side effects.

PhenQ hunger suppressant is FDA-approved. Suitable for men and women without a prescription. Doesn’t matter if you’re a vegetarian or vegan.

How PhenQ Suppresses Your Appetite and Helps You Lose Weight

PhenQ is a weight loss supplement that contains a blend of natural ingredients that help suppress your appetite, burn stored fat, and block new fat production. It also helps to increase your energy levels and improve your mood.

Phenq targets your weight loss in 5 different ways:

  • Burn stored fat by boosting your body’s metabolic and thermogenic rates
  • Suppress your appetite to eat less and cut calories
  • Block fat production to stop excess weight gain
  • Improve your mood and energy levels for hassle-free weight loss

The cool thing is that they offer discounts on their products. For every two PhenQ appetite suppressants you buy, you get one free. They also offer a 60-day money-back guarantee on all orders.


Can Phentermine Make You Tired and Sleepy?

Phentermine is typically known as a stimulant that can increase alertness and energy levels. However, some individuals may experience side effects, including insomnia or disrupted sleep patterns. In such cases, the lack of sleep can lead to tiredness and sleepiness.

Why Does Phentermine Make Me Sleepy and Tired?

Phentermine’s primary mechanism of action is to stimulate the release of certain neurotransmitters, like norepinephrine, which can increase alertness. However, for some people, the stimulating effects may not outweigh the side effects, and they may feel tired or sleepy, possibly due to individual variations in how their bodies respond to the medication.

What’s the Difference Between Phentermine (Adipex) and Duromine?

Phentermine Adipex and Duromine are both brand names for the same active ingredient, Phentermine. However, Adipex is the brand name used in the United States, while Duromine is the brand name used in Australia, New Zealand, and some other countries.

Can Duromine Make You Tired?

Duromine can cause side effects such as fatigue and drowsiness. If you experience these symptoms, it is recommended to speak with your doctor.

Why Does Phentermine Make Me Calm?

Not everyone will experience this effect of calmness when taking Phentermine, but this is due to the way it interacts with their unique brain chemistry.

Why Doesn’t Phentermine Give Me Energy?

While phentermine is generally considered a stimulant that should increase energy levels, individual responses to medications can vary. If phentermine doesn’t give you the expected energy boost, it could be due to factors like individual tolerance, dosage, or interactions with other medications you might be taking

Final Thoughts on Phentermine Fatigue

So yeah, Phentermine may cause you to feel tired, and fatigued. But not sleepy.

However, there are alternatives that do not have any side effects. Always speak with your doctor if you are experiencing any adverse effects while taking Phentermine.

Regardless, I wouldn’t recommend you – as an athlete or fitness enthusiast – to go looking for an appetite suppressant like Phentermine because you can only get through a prescription from a doctor. It is a controlled substance. This will make it a bit difficult to get without going through an illegal means.

This is one of the reasons why a replacement like PhenQ was manufactured in the first place. Check out their official website for more information. They ship worldwide.

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