What are the side effects of Phentermine?

by Marixie Ann Obsioma, MT, undergrad MD on June 12, 2024
Last updated on June 12, 2024

Obesity looms large as a hefty challenge today, hitting folks’ health where it hurts – in both body and wallet.

illustration of white pills

According to the latest data and statistics from the Centers for Disease Control and Prevention (CDC), over 93 million adults in the US are obese (1). Most of the leading causes of premature death like type 2 diabetes, stroke, heart diseases, and cancer, are related to obesity (1). Even more alarming is that the prevalence of obesity is also high in kids and adolescents, especially among 12-19 years old (2).

Eating a balanced diet and being physically active are among the best ways to control obesity and reduce your risk of developing chronic diseases. These lifestyle changes should be enough to keep you healthy and keep off excess fat. However, these do not work for some people; hence they resort to the use of weight loss medications. One of the most popular drugs is Phentermine.

Phentermine is a weight loss drug commonly prescribed by doctors to treat patients suffering from severe obesity. It works by suppressing one’s appetite and increasing the amount of energy being used by the body (3). Phentermine is usually prescribed for a short time only, approximately 3-6 weeks, as an adjunct to diet and exercise (4). Though it is generally safe when taken correctly, some people still experience adverse reactions to phentermine.

Below, let’s go over the most common side effects you may wish to be aware of.

What are the most common side effects of Phentermine? How will you treat them?

Listed below are some of the most common (but minor) side effects of using Phentermine. Though most may go away on their own, there are several things that you can do to relieve them early.

1. Dry Mouth

This is perhaps the most common complaint of Phentermine users. Phentermine can disrupt the salivary glands, reducing the production of saliva (5). While some people are okay with a dry mouth in return for weight loss, others find it very uncomfortable. In fact, this condition may worsen if not given proper attention. One may have bad breath, tooth decay, sore gums, and mouth ulcers (5, 6).

What to do?

To cope with this side effect, you have to increase your daily water intake. Always keep a bottle with you and take frequent sips to keep yourself well hydrated. The recommended daily intake is around 8 glasses (6). It’s good if you can drink more.

Do not drink alcohol and coffee if you have a dry mouth (7). These two acts as diuretics and may worsen the condition.

Chewing sugar-free gums can help induce saliva production (5). Fruits and vegetables with high water content like watermelon and cucumber are highly recommended too (6).

Our tastes buds are likewise inhibited by a dry mouth. Sprinkling a small amount of cayenne pepper on your food can help stimulate the glands to produce saliva, tantalize taste buds, and enhance metabolism (6).

Some users also use mouthwash specifically formulated to treat dry mouth. However, if things are not improving and this side effect is already taking a serious toll on your oral health, consult your physician.

2. Dizziness

Dizziness is yet another common complaint of Phentermine users. The exact mechanism as to how this drug causes dizziness is not well established yet, but experts are relating it to the increased sympathetic nervous activation. There is constriction of the blood vessels, affecting blood flow to the brain (5).

Dizziness may also be due to diet restriction. If your blood sugar level is too low, that can make you feel dizzy (8).

What to do?

To minimize light-headedness, eat small frequent meals to keep your sugar level steady. Include iron in your diet. Eat more eggs, meat, leafy vegetables, and whole grains. Iron is well absorbed in the body when mixed with vitamin C, so you may want to try kiwis, red peppers, and citrus fruits too (9).

If these natural tips don’t help to control your dizziness, consult with your physician and consider seeking urgent medical care is highly recommended.

3. Insomnia

Phentermine is a central nervous system stimulant and those who are taking this drug, especially at higher doses, may experience insomnia. It likewise disrupts the normal circadian rhythm and causes anxiety or stress, which can also prevent a person from getting a good night sleep (5).

What to do?

The duration of action of Phentermine after administration may range from 4-14 hours, depending on the dosage (10). That said, the side effect may be contingent upon the time of administration. It is best to take this drug early in the morning to avoid insomnia. However, always be sure to follow the instructions provided by your physician or pharmacist.

Others benefited from a massage or a warm bath before bedtime. These can help you relax and induce sleep. A cup of tea or a glass of warm milk may also be helpful.

Keep your bedroom setup conducive for sleep. Make your bed comfortable and set the right room temperature.

4. Diarrhea

Users of Phentermine may experience increased frequency of bowel movements due to modified concentrations of gut-bacteria and enhancement of gastric motility (5). But diarrhea isn’t a healthy way to reduce weight as it may lead to severe dehydration and decreased dietary nutrient absorption (11).

What to do?

Drink a lot of water and eat foods rich in fiber. Some users may experience diarrhea during the first few weeks of treatment with Phentermine but subside after that. Your stomach may take time to adjust to the drug itself or to the additives present in its coating.

If the diarrhea is too severe, take an anti-diarrheal drug immediately with the consent of a medical professional.

5. Restlessness

Phentermine, as a stimulant, can increase physical and mental arousal. This makes one hyperactive and energized (5). However, if it lasts too long, it may start to interfere with your ability to stay focus, calm, and productive.

What to do?

You can try to combat restlessness by doing relaxation activities. Others find the use of calming essential oils very helpful. You may also ask your doctor for a safe substance that can induce relaxation.

6. Vomiting

Following dizziness, Phentermine users may also complain of vomiting. In some cases, this also serves as a sign of allergic reaction, especially if accompanied by rashes and swelling. One may also suffer from vomiting if he or she introduced a high dosage of Phentermine at first use (5).

What to do?

Just like diarrhea, vomiting can cause dehydration and decreased levels of body nutrients. It is therefore vital to keep yourself hydrated and eat nutrient-packed foods. Try to consider decreasing the dose too until your body becomes used to it. Any modifications of your dosage should only be done with the help of your doctor.

7. Hair Loss

Not everyone experiences hair thinning or loss, but it is generally observed after months of regular Phentermine use. Though the exact mechanism remains to be unknown, alopecia is being linked to stress and the excessive reaction of the sympathetic nervous system. The body’s immune system attacks its own hair follicles, causing hair loss. Experts are also looking into nutritional deficits as a cause of hair loss (5).

What to do?

Patients suffering from hair loss due to Phentermine use have found great relief by lowering their dosages. Others say that vitamins and supplements can offset this side effect too. But if the hair loss is becoming bothersome or severe, do not hesitate to visit a doctor.

8. Muscle Cramps

Some users of Phentermine complain about muscle cramps. This commonly occurs in the back, legs, and arms. There are several ways by which this drug can cause cramps. The most popular culprits include electrolyte depletion, dehydration, and excessive muscle tension (5).

What to do?

Drink a lot of water. Make sure that you are also getting a good supply of sodium, potassium, magnesium, and calcium. If cramps continue despite water and electrolyte treatments, check your muscles. If there is tension, try relaxing exercises such as stretching and yoga. You may also consider decreasing your dosage. Seek help from a doctor and ask if you can have an anxiolytic drug for support.

9. Sexual Dysfunction

Users of Phentermine may experience changes in sexual desire. Some users report experiencing a significant increase in libido brought about by the induced catecholamine release. However, not everyone taking this drug reports the same effect. Others get a reduction in sexual drive or impotence. This can be attributed to stress and anxiety or the excessive sympathetic reaction, which interferes with a man’s ability to keep an erection. What’s more alarming are the reports saying that Phentermine can cause a temporary shrinkage of the genitals (5).

What to do?

It is highly recommended to tweak the dose of Phentermine to help correct sexual dysfunction. Get enough rest and sleep to limit stress.

What are the serious side effects of Phentermine?

In rare cases, Phentermine can cause serious side effects.

1. Palpitations and Hypertension

Phentermine, as a stimulant, can cause a serious increase in your blood pressure by constricting the peripheral blood vessels, making your heart work harder. This can cause irregular, strong, and rapid heartbeats known as palpitations (12).

Any problems with the heart should always be evaluated by a cardiologist. If he or she says that you are at high risk of developing a heart attack or stroke, you must stop taking Phentermine immediately. If the palpitations are just mild, you may consider decreasing the dosage. If needed, ask your doctor for a safe anxiolytic drug to activate the parasympathetic nervous system and fight palpitations. Beta blockers can likewise be given to control blood pressure.

2. Pulmonary Hypertension

As mentioned above, Phentermine can cause high blood pressure, including in the pulmonary arteries. In a study conducted by Woo-Dae Bang et al, the researchers were able to link the use of Phentermine with a case of pulmonary hypertension. Their subject, a 29-year old woman, was admitted to the hospital due to shortness of breath. She was diagnosed with pulmonary hypertension. Based on medical history, they found out that 3 months before her admission, she took Phentermine for 5 weeks (13). Though not conclusive yet, this remains to be an important basis to examine the effects of Phentermine under different doses further.

This is one of the most serious side effects of Phentermine. If you experience shortness of breath, immediately stop taking the drug and go to the nearest hospital for medical help.

3. Allergic Reaction

Allergies to Phentermine are uncommon, but not impossible. This is commonly due to an interaction with another drug. You may experience skin redness, itchiness, swelling, shortness of breath, and vomiting (5).

Any sign of allergic reaction should be taken seriously. Some allergic reactions could even be fatal. If you notice any of these signs, stop taking Phentermine and get medical help immediately.

4. Blurred Vision

A lot of Phentermine users report experiencing blurred vision. It is believed that this drug increases eye pressure, hence damaging the normal vision. Patients with glaucoma have a higher risk. On the other hand, in normal patients, blurring or doubling of vision may ensue due to excessive sympathetic activation, which includes dilatation of the pupils. This effect is only temporary (5).

If the effect is caused by sympathetic overactivity, reducing the dosage of Phentermine may be helpful. But if you have glaucoma and you think your vision is worsening, visit an ophthalmologist as soon as possible.

5. Depression

Depression can develop among Phentermine users due to excessive sympathetic effects. There is an alteration in blood flow, neural activity, and production of hormones and neurotransmitters. This leaves a user in a constant state of “fight-or-flight” accompanied by irritability and anger. Both of these are related to depression (5).

If you want to continue using Phentermine, it is best to work closely with your physician and/or a psychiatrist. Ask for a dose adjustment to help avoid depression. You may also be given a supportive drug to help stabilize mood.

6. Tremors

Phentermine offers a strong psychostimulatory effect that some users end up having “shakes” or tremors. There is a drastic increase in the levels of catecholamine, which prompts the body to mobilize energy stores and be highly stimulated. Tremors most often occur among users who are taking high doses (5).

Doctors would most likely recommend tapering of doses. However, if the tremors are severe, you may be asked to discontinue the drug. Your doctor will also most likely recommend against taking other stimulants like caffeine that may further increase tremors.


A lucky subset of patients taking Phentermine may be able to finish the treatment course and end up losing weight without experiencing any side effects. However, if you are not one of them and you’re suffering from the excessive psychostimulatory effect of the drug, it may make sense to stop the treatment. Listen to your body. If you cannot tolerate Phentermine, then you may try to look for other weight loss drugs available in the market or just focus more on losing weight naturally.


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