Phentermine and Antibiotics: does it affect antibiotics? (Interactions)

January 8, 2024 |

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If you’re taking phentermine for weight loss, will it be safe to continue using it if your doctor prescribed you antibiotics? What could happen if you use them in combination?

There are limited studies covering the actual effects of using phentermine and different antibiotics together. And of the studies done, there were conflicting results.

For example, no direct drug interactions were found between Amoxil (amoxicillin) and Adipex (phentermine).

However, even if they don’t interact directly, phentermine can increase the negative reactions or side effects of amoxicillin if you don’t eat or drink enough. Considering that phentermine promotes appetite suppression, you are more likely to experience this if you take them together.

Moreover, a 2017 study published in The BMJ (British Medical Journal) Case Reports showed that using phentermine and antibiotics, particularly penicillins, can cause AIN (acute interstitial nephritis), a type of renal failure due to a decline in kidney function from inflammation.

Although the case is considered rare, the study’s researchers recommend that patients and doctors know the risks of this potentially dangerous combination.

Does that mean you should stop taking phentermine if your doctor prescribes you antibiotics? What are the other possible effects of using these two in combination?

Continue reading to learn more below.

Can You Take Phentermine and Antibiotics Together?

There are actually conflicting answers to this question.

Some studies show that phentermine doesn’t have any interaction with most antibiotics, but others also show that using the two in combination can lead to serious medical conditions, including drug-induced renal failure called AIN (acute interstitial nephritis).

According to the researchers of the 2017 study mentioned above, any medication actually has the potential to cause AIN.

However, the case they reported happened to a patient who took phentermine and antibiotics together. So, it’s proof that this combination could be dangerous, especially because AIN leads to renal failure.

AIN is a serious immune-mediated condition that can be treated but could also lead to kidney failure in rare cases.

The case study’s subject experienced the following symptoms for around 5 weeks:

  • Lethargy
  • Nausea
  • Vomiting
  • Left-sided lower abdominal pain

What’s alarming is that the patient had already stopped using phentermine when the symptoms developed and she experienced a weight plateau. However, the symptoms didn’t go away, so she sought treatment to alleviate her condition.

While her experience might not happen to all, it’s good for you to put it in mind if you’re given antibiotics when you’re also taking phentermine.

Be sure to tell your doctor about any medication you’re taking to avoid any dangerous interactions or serious side effects.

Your doctor might also monitor your renal activity if you can’t safely stop using phentermine or antibiotics to avoid complications.

IMPORTANT: Don’t stop using phentermine without consulting your doctor. It can lead to the following serious withdrawal symptoms, especially if you have been taking it at higher doses for an extended period:

  • Seizures
  • Tremors
  • Nerve damage
  • Stroke
  • Congestive heart failure
  • Cardiac arrest

Cardiac arrest leads to loss of heart function and can be fatal.

Do Antibiotics Interfere With Phentermine’s Effects?

Some studies show that ciprofloxacin (a kind of antibiotic) can decrease the metabolism of phentermine. This has mixed effects, with some good and some bad consequences.

As it stays longer in your system, phentermine side effects can also become prolonged and amplified. That isn’t a good thing, especially considering that phentermine can affect your heart, leading to possible heart disease.

In some ways, it can also be good because the more it stays in your body, the longer its effects might be. So, if phentermine isn’t metabolized immediately, then you can feel its effects (such as appetite suppression) for longer.

However, a drug’s efficacy can also become reduced if its metabolism is reduced. So, it follows that phentermine’s effects on your appetite and energy might also be affected.

Since you can’t be sure which of these effects will happen to you, it might be a good idea not to mix them together.

Does Phentermine Affect Antibiotics?

Again, studies are limited and the results can be conflicting. But some show that the oral phentermine-topiramate can alter the drug metabolism of amoxicillin-clarithromycin-lansoprazole oral.

As explained above, altered drug metabolism can reduce its efficacy. So, phentermine might also reduce the effectiveness of antibiotics.

It’s very important that an antibiotic’s effects aren’t reduced because if that happens and the pathogens (such as bacteria) it’s supposed to target can survive, it could lead to antibiotic resistance. Once that happens, you might need stronger antibiotics to fight against common infections.

Phentermine And Antibiotics: Overdose, Abuse, & Other Serious Risks


Antibiotics overdose is rarely fatal but can be uncomfortable because it often leads to stomach upset and diarrhea, with or without a stomach ache.

On the other hand, phentermine overdose can be dangerous!

The most common symptoms of an acute phentermine overdose include the following:

  • Rapid breathing
  • Abdominal cramps
  • Tremors
  • Confusion
  • Restlessness
  • Panic
  • Hallucinations
  • Depression
  • Fatigue
  • Irregular heartbeat
  • Either hypotension (low blood pressure) or hypertension (high blood pressure)
  • Diarrhea
  • Vomiting

Chronic overdose or chronic intoxication symptoms can include:

  • Irritability
  • Hyperactivity
  • Psychosis (delusions or hallucinations)
  • Marked insomnia (severe sleeping problems)
  • Severe skin lesions

Phentermine overdose can be deadly, with some severe cases leading to convulsions, coma, or even death.

If you suspect that you or someone you know is having an overdose, call emergency services or the Poison Helpline at 1-800-222-1222 immediately.

You should monitor for any of the symptoms listed above.

However, the good news is that, unlike the phentermine and alcohol combination, taking phentermine and antibiotics together doesn’t appear to increase their likelihood of amplifying each other’s overdose risks and symptoms.

Side Effects

As discussed above, however, taking antibiotics might not have direct interactions with phentermine but their combination could lead to an increased risk of experiencing side effects due to possible altered metabolism.

Watch out for the following phentermine side effects if you’re also taking antibiotics:

  • Dizziness
  • Headache
  • Feeling nervous
  • Restlessness
  • Constipation or diarrhea
  • Trouble sleeping
  • Dry mouth
  • Bad taste in your mouth
  • Reduced interest in sex
  • Erectile dysfunction


Phentermine and antibiotics can both be abused but in different ways.

Phentermine is a stimulant classified as a scheduled drug because of its potential for dependence and drug abuse. It can produce a similar effect as its cousin, amphetamine. So, you might feel “high” while taking phentermine.

The abuse of antibiotics isn’t the same as that of phentermine. They’re not abused recreationally, nor do they lead to drug dependence and cause serious withdrawal symptoms.

Although antibiotics don’t make you feel high because they aren’t stimulants, using them even if they aren’t needed is considered a form of drug abuse.

Antibiotics abuse is actually common and can lead to resistant bacteria that are harder to treat and might require stronger medications.

What’s A Better Weight-Loss Drug Than Phentermine?

Phentermine might be approved by the FDA as a weight-loss drug but it doesn’t automatically mean that it’s good for you.

As you can see above, it’s a drug that can have many serious side effects and dangerous risks. What’s worse is that you’re at risk of these dangers whether or not you’re taking antibiotics!

That’s why we recommend using PhenQ, instead.

PhenQ is an appetite suppression dietary supplement that can also give you an energy boost, similar to what phentermine can do, but without fear of experiencing serious complications and side effects.

This dietary supplement doesn’t contain phentermine. Instead, it uses the patented a-LACYS RESET as the main ingredient that can boost your slimming efforts naturally.

FAQs on Phentermine and Antibiotics

Can You Take Phentermine With Other Medications?

Phentermine can interact with many different medications, so it’s important to always tell your doctor if you’re either taking it or any other drug so they can adjust your dose or choose other options.

What Medications Should Not Be Taken With Phentermine?

Drugs that are known to negatively interact with phentermine:

  • CNS (central nervous system) depressants such as benzodiazepines, barbiturates, and sleep medications
  • Desmopressin
  • Linezolid
  • Procarbazine
  • Tranylcypromine
  • Selegiline
  • Rasagiline
  • Amphetamine
  • Epinephrine
  • Antacids
  • Dopamine
  • Amantadine
  • Antihypertensives (such as beta-blockers)
  • SSRIs – selective serotonin reuptake inhibitors such as Paxil (paroxetine), Prozac (fluoxetine), Luvox (fluvoxamine), and Zoloft (sertraline)
  • MAOIs – monoamine oxidase inhibitors such as Marplan (isocarboxazid) and Nardil (phenelzine)
  • Oral contraceptives (might cause irregular bleeding)

Can I Take Phentermine With Amoxicillin?

Phentermine doesn’t appear to have direct interaction with amoxicillin. However, some studies show that if you don’t eat or drink enough while taking the combination, amoxicillin could increase phentermine’s negative reactions.

Also, penicillins like amoxicillin can increase your risk for AIN (acute interstitial nephritis), which can be dangerous because it could lead to kidney failure.

Can You Take Phentermine And Ibuprofen Together?

Ibuprofen drugs such as Advil don’t appear to interact with phentermine. However, it’s still best to consult your doctor before taking them together.

Wrap up

Phentermine is a weight-loss drug that has been linked to several serious health complications.

If you are taking phentermine, it is important to be aware of the risks and talk to your doctor about any potential interactions with other medications you may be taking.

Antibiotics can also cause serious side effects, so it is important to be aware of the risks and always follow your doctor’s instructions carefully.

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