Why Does Phentermine Cause Back Pain? (reasons & treatment)

January 9, 2024 |

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Around 26.32% of phentermine users might experience back pain.

But it’s usually not a major cause of concern unless the pain persists or is accompanied by other, more serious symptoms such as dark or bloody urine, painful urination, and/or stomach pain.

If you’ve been taking phentermine, persistent back pain accompanied by these symptoms could signal a problem with your kidneys. It could be serious and possibly due to drug-induced AIN (acute interstitial nephritis), a condition that can be treated but isn’t always curable.

Should you experience sharp back pain, particularly with stomach pain and bloody urine with phentermine use, head straight to the ER or call your doctor.

Remember that you can’t just stop taking phentermine, especially if you are taking high doses because it could also lead to dangerous withdrawal symptoms!

Are there other reasons for back pain due to phentermine aside from AIN? What are the treatment options for back pain due to phentermine use, is there a way to prevent this from happening, and how long will it last?

Continue reading to find answers to these questions below and also learn about a safer, healthier alternative to phentermine that doesn’t cause back pain.

Reasons For Back Pain While Taking Phentermine

Back pain isn’t a very common side effect of phentermine compared with its effects on the heart and brain. However, back pain might be a symptom of a more serious condition.

So, it’s important not to ignore back pain if you’re taking phentermine.

Here are some of the possible reasons:

  • AIN (acute interstitial nephritis)
  • Kidney stones
  • Rhabdomyolysis
  • Kidney deterioration due to high blood pressure

When diagnosing the main cause of back pain from phentermine use, your doctor will likely ask about the other symptoms that you experience and the exact location of the painful part on your back.

Proper diagnosis is important to find the best treatment for your condition.

Let’s dive into each possible reason phentermine causes back pain:

1. AIN (Acute Interstitial Nephritis)

Phentermine can induce AIN (acute interstitial nephritis). This immune-mediated condition is characterized by inflammation or swelling in the kidney interstitium, a fluid-filled space supported by a mesh of connective tissues. The interstitium is part of your kidney’s filters.

AIN has several manifestations, but can also show in the form of severe pain in the lower back.

People with this condition can also experience the following symptoms:

  • Stomach pain
  • Fever
  • Nausea
  • Vomiting
  • Rash
  • Increased urination frequency but with a reduced amount of urine
  • Pain during urination
  • Dark or bloody urine
  • High blood pressure
  • Drowsiness
  • Confusion
  • Weight gain
  • Swelling throughout your body

Pain in your flank area (between your lower ribs and hips) is also possible, although it’s uncommon. This condition might also lead to breathing difficulties.

Most cases of AIN caused by phentermine or other drugs are temporary and reversible. However, early treatment is important to reduce the risk of permanent kidney damage.

So, it’s best to see your doctor or go to the ER if you experience severe back pain while taking phentermine.

Take note of any symptoms you experience and be sure to inform your doctor about them.

2. Metabolic Acidosis & Kidney Stones

Phentermine use can lead to metabolic acidosis which can cause kidney stones.

Signs and symptoms of kidney stones can include the following:

  • Pain in the lower back or side
  • Pain when urinating
  • Blood in urine

Although it’s rare to develop kidney stones from phentermine use, this side effect tends to have a severe expression if you do experience it.

Because metabolic acidosis can lead to kidney stones, it can indirectly cause back pain.

Other symptoms of metabolic acidosis include:

  • Fast breathing
  • Fatigue
  • Loss of appetite
  • Irregular heartbeat
  • Loss of consciousness

3. Rhabdomyolysis

The use of phentermine can also lead to rhabdomyolysis, a condition that could manifest as a radiating pain on your lower back, thighs, and shoulders.

These are the most common locations for pain due to rhabdomyolysis.

The problem with rhabdomyolysis, however, is that it can be difficult to diagnose because it can have different symptoms among patients, depending on its main cause.

It might also occur in just one area or your whole body.

Be sure to tell your doctor about any symptoms you experience, including the following:

  • Muscle weakness or trouble moving your arms and/or legs
  • Dark red or brown urine
  • Decreased urination
  • Nausea
  • Vomiting
  • Rapid heart rate
  • Fever
  • Confusion
  • Dehydration
  • Abdominal pain
  • Losing consciousness

Some patients don’t show any signs of back pain or other muscle-related symptoms.

4. Blood Pressure-Induced Kidney Problem

Phentermine can elevate blood pressure, which can hasten kidney deterioration. As you can see above, problems with your kidneys can lead to lower back pain.

How Long Does Back Pain Last With Phentermine Use?

Man with back pain

The duration of your back pain will depend on several factors, including which organ or part of your body was directly affected by your phentermine use.

Kidney problems can last for weeks, even months, and might not go away unless you receive treatment.

In rare cases, you might need surgery, including the following:

  • Fasciotomy (for rhabdomyolysis)
  • Surgical removal of kidney stones

How To Prevent Back Pain While Using Phentermine

Back pain from phentermine use is often due to a problem with your kidneys.

That’s why it’s important to protect your kidneys from damage and reduce your risks of experiencing the medical conditions mentioned above.

One way to do that is to drink plenty of water. Increased water intake can make you urinate more often, which can help prevent problems in your kidneys.

It’s also a good idea to avoid mixing phentermine and alcohol because both can affect your kidneys. Avoiding this mix can also indirectly help prevent back pain with phentermine use.

Sometimes, the best way to prevent this kind of pain is to quit using phentermine.

When To See A Doctor For Back Pain With Phentermine

Most conditions that cause back pain with phentermine use are actually severe. They often manifest due to a serious underlying condition.

That’s why it’s important to see your doctor if you experience back pain while taking this obese drug. Your doctor will determine the best treatment plan and can also help you decide whether it’s better to just stop using it instead.

Go to the ER for severe back pain, especially if it’s accompanied by bloody or dark urine, rapid heart rate, rash, fever, and/or severe stomach pain.

Inform your loved ones that this drug can also lead to loss of consciousness. They should immediately call for emergency services or rush you to the ER if that happens.

Treatment For Back Pain With Phentermine Use

Quitting Phentermine

The treatment options will depend on the reason for your back pain with phentermine use.

However, because your back pain and these medical conditions might be due to phentermine, it makes sense that quitting this obesity drug could be your main treatment option.

Warning: Don’t stop taking phentermine without your doctor’s advice. Serious withdrawal symptoms can include severe tiredness and depression.

Your doctor will weigh the risks and might consider gradually reducing the phentermine dose to avoid these serious withdrawal symptoms unless your phentermine-induced medical condition is serious.


Because phentermine-induced AIN is caused by your immune system’s reaction to this obesity drug, your condition might improve once you quit using this product.

Aside from drug removal, this condition can also be treated with anti-inflammatory medications.

Lifestyle changes, such as following a low-protein, low-salt diet can also help improve your symptoms.

Severe cases might require dialysis to support kidney function or, worse, a transplant in the event of kidney failure.

Metabolic Acidosis & Kidney Stones

Metabolic acidosis happens when too much acid is produced by your body or your damaged kidneys can no longer remove them. So, treatment can include raising your blood pH using oral or intravenous sodium bicarbonate.

Your doctor might also consider surgical removal of your kidney stones or provide you with medications to try to dissolve them.

In rare, severe cases, you might need dialysis or renal replacement therapy.


Possible treatment plans for rhabdomyolysis:

  • Management of electrolyte abnormalities (e.g., calcium, potassium, or phosphorus) to protect your heart and other organs
  • IV (intravenous) fluids to help maintain urine production, which can help prevent kidney failure
  • Fasciotomy (a surgical procedure to prevent loss of circulation or relieve pressure in the affected area)
  • Dialysis treatment is rare but your doctor might consider it to help your kidneys filter waste products while you’re recovering
  • Changes or limitations in your diet might also help alleviate the symptoms

Blood Pressure-Induced Kidney Problem

Management of your blood pressure can also help prevent damage to your kidneys. So, make sure to take your hypertension maintenance medications, with your doctor’s approval.

Phentermine Alternative without back pain (safer diet pill)

Worried about back pain and the other serious complications from phentermine use? You might want to switch to the PhenQ appetite suppressant.

This weight-loss dietary supplement doesn’t cause back pain or the other side effects that phentermine is known for, yet it can still work as effectively.

PhenQ helps you crush cravings, improve your metabolism, shed off excess fats, and reduce fat accumulation. It even helps balance your moods and energy levels to motivate you to try harder.

This unique formula contains the patented a-LACYS RESET ingredient which can boost your slimming efforts naturally.

So, in choosing PhenQ, you get all the benefits of phentermine without worrying about back pain and other serious side effects.

Final thoughts

Phentermine is a weight-loss medication that can help you shed those extra pounds, but it’s not without its side effects. Some of the more common side effects include dry mouth, insomnia, and constipation.

However, one of the lesser-known side effects is back pain.

If you experience any unusual or concerning symptoms while taking phentermine, it’s important to seek medical help right away. In some cases, these symptoms could signal a problem with your kidneys, drug-induced AIN, or another underlying health condition.

If you’re struggling with weight loss, don’t give up!

As there are plenty of other options out there for you, including the all-natural diet pill PhenQ.

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