How Long Do Antibiotics Stay in Your System?

There is no one-fits-all answer to the question of how long antibiotics stay in your system because every person and medication is unique and there are several factors that affect how long a medication will stay in a person’s body.

Let’s dive deeper into antibiotics and the factors that affect how long they will be present in a person’s body. 

Table of Contents

What Are Antibiotics?

Antibiotics are a type of medication that is primarily used to treat or prevent bacterial infections. It is important to remember that antibiotics are only useful in treating bacterial infections, they will not be effective against viral or fungal infections. 

How Do Antibiotics Work?

Depending on the specific antibiotic, it either kills the bacteria cells or makes it difficult for the bacterial cells to continue to grow and reproduce. Both methods will aid in eliminating the bacteria from a person’s body.  

What Are Antibiotics Used For?

Antibiotics are used to treat infections and illnesses that are caused by bacteria. If an illness is caused by a virus, fungi, or parasite, antibiotics won’t be an effective treatment.

Types of Antibiotics

There are nine classes of antibiotics which are, 

  • Penicillins (example: amoxicillin)
  • Macrolides (examples: azithromycin, erythromycin)
  • Cephalosporins (examples: cephalexin, cefdinir)
  • Fluoroquinolones (examples: ciprofloxacin, levofloxacin)
  • Beta-lactams with increased activity (examples: amoxicillin/clavulanate, ceftazidime/avibactam)
  • Tetracyclines (examples: tetracycline, doxycycline)
  • Trimethoprim-sulfamethoxazole 
  • Urinary anti-infective (example: nitrofurantoin) 
  • Lincosamides (example: clindamycin)

How Long Do Antibiotics Stay In Your System?

There is no concrete answer to this question. It can be estimated based on a medication’s half-life but there is no exact answer. Every medication has a unique half-life which is the approximate amount of time it takes for the medication to reduce by 50% in the body. 

How long a specific medication stays in your body will depend on several factors such as:


As humans grow older their response to medications is altered, this is caused by changes in gene expression, metabolism, and cell damage. Older age is associated with greater levels of drug concentrations in the blood, reduced effectiveness of medications, and greater risks of adverse reactions to medications. 

Medical Conditions

Disease, infection, and inflammation can slow down drug metabolism which will increase the amount of time a medication is in the body. 

Organ Function 

People who have decreased kidney, liver, or heart function, will have medications stay in the body for a longer amount of time. The kidneys and liver aid in filtering the blood and the heart aids in circulating the blood. When these functions are compromised it will make drug elimination longer. 

Medication Dosage 

The dosage or amount of the medication given will affect the amount of time a medication stays in the body. Larger dosages will generally take longer to leave the body. 

Diet and Other Medications 

There are food-drug interactions and drug-drug interactions that either speed up or slow down the speed at which a medication such as an antibiotic will stay in the body. 


Research has shown that when men and women are given the same dosage of a medication, the women have higher concentrations of the medication in their blood and it takes longer for the medication to leave their body. 

Body Weight

The relationship between weight and how fast a medication is eliminated from the body is not well understood. It is thought that factors such as body fat percentage, muscle mass percentage, water levels, and blood volume affect the amount of time a drug is in the body. 


People can have many different responses to the same medication which helps prove that genetics play a role in medication metabolism. It is believed that genetic factors account for 20% to 95% of patient differences.  

How the Medication Was Administered

A medication can be administered by several different routes such as orally, intravenously, intramuscularly, and more. Each route will affect the medication’s rate of absorption and elimination. 

Pharmacokinetics and Medication Half-life

There is an entire branch of pharmacology that studies the movement of drugs through the body called pharmacokinetics. It looks at factors such as the absorption, distribution, metabolism, and elimination of drugs in the body. One of the factors they look at is a medication’s half-life. 

Every medication has a unique half-life which is the approximate amount of time it takes for the medication to reduce by 50% in the body. Each medication is different but it takes an average of 5-7 half-lives for a medication to be completely gone from the body.

How Long Do Common Antibiotics Stay in Your System?

Considering the factors discussed previously, the average amount of time a medication will stay in your system can be estimated by multiplying a medication’s half-life by 5 – 7.  

  • Penicillin:
    • Half-life = 0.5 – 0.6 hours
    • Estimated time for penicillin to leave the body = 2.5 – 4.2 hours
  • Amoxicillin:
    • Half-life = 0.7 – 1.4 hours
    • Estimated time for amoxicillin to leave the body = 3.5 – 9.8 hours
  • Metronidazole:
    • Half-life = 8 hours
    • Estimated time for metronidazole to leave the body = 40 – 56 hours
  • Ciprofloxacin:
    • Half-life = 3 – 5 hours
    • Estimated time for ciprofloxacin to leave the body = 15 – 35 hours
  • Cephalexin:
    • Half-life = 0.5 – 1.2 hours
    • Estimated time for cephalexin to leave the body = 2.5 – 8.4 hours
  • Doxycycline:
    • Half-life = 15 – 25 hours
    • Estimated time for doxycycline to leave the body = 75 – 175 hours

Common Side Effects of Antibiotics

The side effects of antibiotics can vary greatly. If you are having any side effects from an antibiotic you should notify the healthcare provider who prescribed the medication. They will give you further instructions. 

Common side effects of antibiotics include:

  • Nausea
  • Vomiting
  • Diarrhea
  • Cramps or abdominal pain
  • Fever
  • Bloody stool
  • Yeast infections

If you experience any severe reactions you should seek emergency medical care.  

When to See a Doctor?

If you are experiencing an illness that you believe could be bacterial and might require antibiotics you should see a healthcare provider. They will assess you, ask you questions, and give you recommendations. 

Remember that antibiotics are only effective against bacterial infections. If the healthcare provider does not think you are suffering from a bacterial infection, they will not prescribe antibiotics. This helps prevent antibiotic resistance. 

How Can Drhouse Help You?

Technology has now allowed patients to connect with healthcare providers quicker than ever before. Telehealth has grown tremendously in the last few years. At DrHouse you can connect with a board-certified online doctor in as little as 15 minutes. You can even schedule an in-person visit with one of their doctors. This is convenient when it comes to getting medical treatment faster. 

It will take some primary doctor’s office days to get you in for an appointment. With DrHouse you could see a doctor in less time than it takes you to cook a meal or take a shower. You won’t even have to leave your house. This will save you time and allow you to get quick treatment for your medical concerns.

Key Takeaways

Antibiotics are a type of medication that fights bacterial infections by killing the bacteria cells that have invaded a person’s body or by making it difficult for the bacterial cells to continue to grow and reproduce. 

Antibiotics are only used to treat infections and illnesses that are caused by bacteria. If an illness is caused by a virus, fungi, or parasite, antibiotics won’t be an effective treatment. There is no concrete answer to the question of ‘how long do antibiotics stay in your system. 

How long a medication stays in your body will depend on several factors such as age, medical conditions, diet, weight, gender, organ function, other medications, genetics, medication administration, and medication characteristics. 


DrHouse articles are written by MDs, NPs, nutritionists and other healthcare professionals. The contents of the DrHouse site are for informational purposes only and are not a substitute for professional medical advice, diagnosis, or treatment. If you are experiencing high fever (>103F/39.4C), shortness of breath, difficulty breathing, chest pain, heart palpitations, abnormal bruising, abnormal bleeding, extreme fatigue, dizziness, new weakness or paralysis, difficulty with speech, confusion, extreme pain in any body part, or inability to remain hydrated or keep down fluids or feel you may have any other life-threatening condition, please go to the emergency department or call 911 immediately.

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