Phentermine and Nausea

Phentermine nausea affects about 7% of patients, and can range from a mild annoyance to debilitating discomfort ( 1 ). It has different etiologies, but this bothersome phentermine side effect most often strikes in the first few days or weeks of treatment. Most cases of mild stomach upset resolve on their own with at-home management.

Contact a doctor if phentermine nausea is severe, progresses to vomiting or persists for more than 48 hours.

Why Does Phentermine Cause Nausea?

young woman covering her mouth during a wave of nausea
Under 10% of patients surveyed experienced any phentermine nausea or vomiting

Stimulants cause nausea for a variety of reasons, including excessive hunger, the pills’ ingredients, or an adverse drug reaction. Phentermine nausea, in particular, may be related to:

1. CNS Stimulation

Nausea and vomiting are listed as side effects of phentermine, as well as other stimulant medications like Ritalin and Adderall.

Phentermine helps users lose weight by exciting the central nervous system (CNS) to achieve a constant state of activation, similar to “fight or flight”. However, one unfortunate consequence of this stimulation is nausea, or even vomiting ( 2 ).

The cascade of physical, hormonal and emotional changes associated with this heightened state precipitate nausea, partially because of slowed digestion and food staying in the stomach for longer ( 3 , 4 ).

If you experience nausea after eating on phentermine, this is a likely culprit. Food stuck in the stomach proves a common cause of both heartburn and nausea in adults.

2. Excessive Hunger or Thirst

Growing overly-hungry or thirsty can cause phentermine nausea, especially if you have diabetes ( 5 ).

Queasiness is a common symptom of low blood sugar, which can arise from eating too little or too infrequently. Since phentermine acts as a powerful appetite suppressant, patients forget to eat – or choose to dramatically decrease intake – and experience low blood sugars as a result.

Dehydration can also cause nausea because your cells need water to function properly. When cells in the stomach cannot produce the proteins they need, they signal something is wrong and stop working, which makes you feel nauseous ( 6 ).

Phentermine may affect your need for diabetes medications ( 7 ).

If you use insulin or take an oral hypoglycemic medication like Metformin, discuss any phentermine nausea (or other symptoms of low blood glucose) with your doctor right away.

3. Sensitive Stomach

Taking medications or supplements on an empty stomach can cause nausea and vomiting, especially if you have a sensitive stomach. This is because medicine, supplements, and vitamins all irritate your stomach.

So, when patients take phentermine first thing in the morning or hours after breakfast – as often directed – it can cause phentermine nausea.

4. Drug Reaction or Allergy

Phentermine nausea can also be a sign of an adverse drug reaction or allergy.

Contact a doctor right away if your phentermine nausea is sudden, severe, or accompanied by other worrisome symptoms.

Phentermine Nausea Relief

lemon tea for phentermine nausea relief
Sipping on lemon or ginger tea may help alleviate phentermine nausea

Phentermine nausea strikes most patients in the first week or two of treatment and lasts for anywhere from a few hours to a few days. If you have mild to moderate phentermine nausea, try these common remedies to reduce queasiness:

Drink Water

One of the most common causes of nausea, with or without phentermine, is dehydration. If you have phentermine nausea and you have been skimping on water, drink 1-2 big glasses of water and wait 20 minutes. Sometimes this is all you need make quell the stomach upset.

Prevent the return of this unpleasant sensation by drinking at least 8-10 glasses of water per day. If you feel thirsty, get dizzy or produce dark yellow urine, you’re already dehydrated ( 8 ).

You know you’re properly hydrated if your urine is pale yellow.

Adjust Eating Habits

Eating too much spicy, fried, high-fat or high-fiber food can make stomach problems worse, as can drinking too much caffeine or alcohol ( 9 ). While food does not generally cause nausea, it can make it worse if you are already feeling queasy from the medication.

Interestingly, people who choose high-protein breakfasts tend to have less nausea than those who eat a sugary first meal. Plus, breakfasts with at least 30g of protein have been proven beneficial for weight loss as well ( 11 ).

After breakfast, eat something light every 4-6 hours while awake and stop eating at least 2-3 hours before bedtime. The latter reduces nocturnal acid reflux and related phentermine nausea.

Try a Natural Antiemetic

Another option is to try natural antiemetics – such as ginger, peppermint or lemon – to reduce nausea.

Ginger is one of the most well-known and well-studied homeopathic remedies for nausea and vomiting. The root is has been scientifically proven to reduce nausea related to different causes, including sea sickness, motion sickness and chemotherapy ( 12 ).

Peppermint or lemon may also help alleviate nausea. Peppermint tea or water with a slice of lemon are both effective, at-home treatments for phentermine nausea. Aromatherapy with these scents may also decrease nausea ( 13 ).

Talk to Your Doctor

Talk to a doctor if phentermine nausea persists, worsens, or interferes with your daily routine.

He or she may suggest decreasing, splitting, or otherwise adjusting your dose. If the pill itself is making you nauseous, discuss different phentermine dosage options with your prescribing doctor. Your doctor and pharmacist can also answer questions about which OTC medications you can take to manage phentermine nausea.

Reminder: NEVER adjust your dose or schedule without speaking to a medical professional first.

Back to All Phentermine Side Effects

  1. Members of “Losing Weight with Phentermine” Support Group on Facebook & Forum. (2019, March 13). [User Report of Common Phentermine Side Effects]. Unpublished raw data.
  2. LaCount, L. T., Barbieri, R., Park, K., Kim, J., Brown, E. N., Kuo, B., & Napadow, V. (2011). Static and dynamic autonomic response with increasing nausea perception. Aviation, Space and Environmental Medicine, 82(4), 424-433. PMCID: PMC3137518
  3. Muth, E. R. (2006). Motion and space sickness: Intestinal and autonomic correlates. Autonomic Neuroscience, 129(1-2), 58-66. doi:10.1016/j.autneu.2006.07.020
  4. Folk, J., & Folk, M. (2021). Nausea and Vomiting – anxiety symptoms.
  5. American Diabetes Association. (2018, August 29). Hypoglycemia (Low Blood Glucose).
  6. Miller, K. (2021). 6 Surprising Reasons Why You’re Feeling Nauseous.
  7. U.S. Food and Drug Administration. (2012). Adipex-P (phentermine hydrochloride) capsules label [Brochure].
  8. Mayo Clinic Staff. (2018, February 15). Dehydration.
  9. University of Wisconsin – Madison: University Health Services. (n.d.). Upset Stomach.
  10. CATIE. (n.d.)
  11. Rains, T. M., Leidy, H. J., Sanoshy, K. D., Lawless, A. L., & Maki, K. C. (2015). A randomized, controlled, crossover trial to assess the acute appetitive and metabolic effects of sausage and egg-based convenience breakfast meals in overweight premenopausal women. Nutrition Journal, 14(1). doi:10.1186/s12937-015-0002-7
  12. Ernst, E., & Pittler, M. H. (2000). Efficacy of ginger for nausea and vomiting: A systematic review of randomized clinical trials. British Journal of Anaesthesia, 84(3), 367-371. doi:10.1093/oxfordjournals.bja.a013442
  13. Vann, M. R. (2017, July 25). 4 Natural Remedies for Nausea (J. Laube, Ed.).