Tired on Phentermine


Most people experience a boost of energy, and even jitteriness or a “wired” feeling, while taking phentermine due to its stimulant effects. However, some people experience an opposite phentermine side effect: fatigue.

Instead of a boost, these phentermine users feel sleepy or mentally and physically exhausted. Phentermine fatigue varies in severity and duration.

This article is intended for informational purposes only. We are not doctors, and the information below does not in any way constitute or substitute a professional medical opinion. Always contact a licensed medical professional for medical advice.

Can Phentermine Make You Tired?

woman yawning as a result of phentermine fatigue
Almost 20% of patients report phentermine fatigue

Yes, phentermine can make you tired.

This medication is famous for its energy-boosting properties, but it can leave some users feeling surprisingly tired. Mild phentermine fatigue may be attributable to chronic use, a paradoxical reaction, medication-related insomnia, or lifestyle factors that exacerbate fatigue.


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Feeling a little sleepy is not uncommon, but feeling “zombie-like” or completely exhausted is NOT normal. Weakness or fatigue that makes it hard to carry out everyday activities, or decreases your ability to exercise, may indicate a serious phentermine side effect and should be reported to your doctor immediately.

Talk to a doctor right away if you feel unusually tired or lethargic while taking phentermine or if your fatigue is accompanied by other problems like chest pain, shortness of breath, or swelling in the lower extremities (edema).

Causes of Phentermine Fatigue

About 1 in 5 users report experiencing phentermine fatigue ( 1 ). While severity, duration, and cause vary between patients, there are several common explanations for why phentermine makes you sleepy.

Metabolism of the Stimulant

If you’re taking phentermine and often feel tired in the afternoon, the drowsiness may be associated with the medication’s active ingredient wearing off.

Phentermine hydrochloride has a half-life of 7-34 hours, and most patients feel its stimulant effects for about 10-14 hours ( 2 , 3 ).


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So, some people experience phentermine fatigue if they take the pill first thing in the morning and then remain active for more than 14 hours. Many patients prefer for the medication’s stimulant effects to wane before they go to bed, but if the return of fatigue proves problematic for you talk to your doctor about possibly splitting your daily dose in two.

Chronic Use

Long-term stimulant use can cause feelings of fatigue due to the exhaustion of dopamine supplies.

As a central nervous system stimulant, phentermine promotes the release of dopamine and other catecholamines. However, after an extended period of use, the natural supply of these neurotransmitters can “run dry” and leave you feeling down and fatigued.

If you are experiencing phentermine fatigue a few weeks or months into treatment, talk to your doctor. He or she may reduce your dose or offer other solutions to help you feel less tired on phentermine.

Paradoxical Reactions

Some people have paradoxical reactions to stimulants, which produce fatigue, drowsiness, and reduced energy levels – the exact opposite of the intended effect.

Like more common stimulants such as caffeine or amphetamine, phentermine leaves some patients feeling drowsy and tired from the start. This phentermine fatigue can occur independently or alongside other side effects like insomnia.

Medical experts do not fully understand why paradoxical reactions occur, but evidence indicates that inconsistent responses to stimulants are related to genetic or biological differences between individuals ( 4 ).

woman covering her head with a pillow
Phentermine insomnia at night can translate to phentermine fatigue during the day


Phentermine works as a stimulant to suppress appetite, but it does not only affect appetite – it also stimulates the rest of the brain and body by activating the part of the brain responsible for the “fight or flight” response.


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As a result, when the stimulant is not completely metabolized before bedtime, this weight loss medication can cause insomnia. If you are having trouble falling or staying asleep night after night, daytime phentermine fatigue is a logical result of sleep deprivation.

Click here to read more about phentermine insomnia and how to cope.

Fatigue Associated with Weight Loss

Sleepiness is also a common side effect of weight loss in general. Changes in diet, exercise, and mentality often produce unwanted fatigue.

For example, eating too little of specific nutrients, or too few calories, in general, can contribute to phentermine fatigue. Energy levels particularly suffer if you have low levels of vitamin D, iron, magnesium, B12, folate or too few omega-3 fatty acids ( 5 ). Diets high in saturated and/or trans fats or sugar can also cause or worsen fatigue.

Activity level also affects energy levels. An overly-sedentary lifestyle contributes to phentermine fatigue since your body grows accustomed to needing a very low level of energy.

Finally, if your weight loss journey has proven trying thus far, try to reduce stressors where possible since stress wears you down both mentally and physically.

How to Overcome Phentermine Fatigue

If you are feeling overly tired on phentermine, there are some at-home techniques you can use to start feeling more energetic faster. Here are five easy remedies for phentermine fatigue:

Contact a doctor if phentermine fatigue is severe, persistent, or interfering with your daily life.

1. Drink Water

Real users’ most common advice to others who are feeling tired on phentermine is to drink more water.

Patients often report that they feel sleepy and groggy when they forget to drink enough water, but feel the medication’s energy-boosting power return on days when they are properly hydrated.

This makes a lot of sense since even mild dehydration can zap energy. To minimize phentermine fatigue, drink at least 2-3 liters of water or another calorie-free, caffeine-free beverage each day.


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2. Get Moving

It seems counter-intuitive, but moving around can actually help boost energy.

While you may feel like all you need is a nap, if you have been resting a lot and still feel sleepy, try getting up and moving around. Exercise increases energy in both the short- and long-term.

Regular physical activity helps combat phentermine fatigue by increasing endorphins to boost mood (and energy!) right away and then improving cardiovascular fitness to combat drowsiness long-term. When you’re in better shape, it takes less energy to finish everyday tasks, so staying fit can help you feel more energetic overall.

More, research shows that moderate exercise reduces insomnia, plus decreases symptoms of depression and anxiety, which means that breaking a sweat may also help you sleep better at night ( 6 ).

If you are not in the mood to workout, get your steps in by increasing day-to-day activity. Make a point to take a short walk around the office each hour, clean a little bit in the morning, park farther away from the store, or play with your kid or pet for a few minutes each day.

Aim for 30 minutes of moderate activity on most days.

3. Grab a Healthy Snack

woman tossing an apple in the kitchen
If you’re tired on phentermine, grab a healthy snack to power-up

Eating too much or too little, or consuming a poor-quality diet, can also impact energy levels.

If you suspect that your phentermine fatigue may be related to your diet, re-evaluate your meals and snacks to maximize nutrition and meet calorie goals each day. Aim for a balanced combination of lean protein, healthy fats, and fiber-rich carbs (including beans, fruits, and vegetables) each day.

4. Take a Break

Stress both causes and exacerbates fatigue.

So, when you feel tired on phentermine, it is worthwhile to evaluate whether that exhaustion is at least partially related to stress or insomnia.

Take a nap if you haven’t been sleeping. Meanwhile, consider meditation, deep breathing, journaling, gentle exercise (e.g., walking or yoga), a chat with a friend, or other relaxation techniques to manage and reduce stress.

5. Decrease Dosage

If you suspect that a paradoxical reaction may be responsible for your phentermine fatigue, one option would be to speak to your doctor about reducing your daily dose.

If your doctor rules out other causes of phentermine fatigue, taking less of the medication may help alleviate this unwanted side effect. NEVER adjust your phentermine dose or schedule without talking with a doctor first.

Talk to a doctor if you continue feeling tired on phentermine, or if the fatigue intensifies or interferes with your daily life.

Back to All Phentermine Side Effects

  1. Members of “Losing Weight with Phentermine” Support Group on Facebook. (2019, March 13). [User Report of Common Phentermine Side Effects]. Unpublished raw data.
  2. U.S. National Library of Medicine. (2006, January 12). PHENTERMINE.
  3. Dr. Rick, MD. (2012, April 02). Phentermine 37.5 mg/daily in moring. My question is how does…
  4. Lowry, F. (2012, February 16). Mystery of Psychostimulant Paradox Solved.
  5. Schroeder, M. O. (2015, September 8). Is a Vitamin or Mineral Deficiency Making You Tired
  6. Hartescu, I., Morgan, K., & Stevinson, C. D. (2015). Increased physical activity improves sleep and mood outcomes in inactive people with insomnia: A randomized controlled trial. Journal of Sleep Research, 24(5), 526-534. doi:10.1111/jsr.12297