How Long Do Antibiotics Stay in Your System?

Medically reviewed by Erika Prouty, PharmD

Antibiotics can generally stay in your body for hours or even days, depending on your health status and the duration of your treatment.

Antibiotics are antibacterial substances, such as penicillin, used to treat or prevent infections by killing or stopping the growth of bacteria in or on the body.

Antibiotic therapy can be given in the hospital setting for more severe infections or prescribed by a healthcare provider to treat acute infections. Treatment courses can last long-term (over six weeks) or short-term (three to five days), depending on the severity of the infection.

This article will discuss how long different antibiotics will stay in your body after taking them.

How Long Do Antibiotics Stay in Your System?

Most antibiotics start working right after you take the first dose. Each antibiotic may stay in your body for a different amount of time, as many different classes exist.

Several factors can influence how long an antibiotic stays in your system, including the type of antibiotic, how long you're taking it, and your health.

Some common antibiotics, such as penicillin (e.g., amoxicillin) or ciprofloxacin, can stay in your body for up to 12 to 24 hours after your last dose.

Amoxicillin, for example, is excreted through the kidneys and urine. Most of it is expected to pass through the urine within eight hours of the last dose and should be completely gone by 12 hours.

Other antibiotics can stay in the body much longer. Zithromax (azithromycin), for example, can stay in the body for up to 14 days after the last dose.

If you have kidney problems, clearing the antibiotics from your body might take longer. Additionally, being on a longer-term antibiotic regimen may increase the time it lasts in the body.

Related: Every Antibiotics Question You Have, Answered

What Does Antibiotic Half-Life Mean?

The best way to identify how long a medication will last in the body is by measuring the half-life of a medication.

A half-life is the time it may take for the strength or concentration of the medication to decrease by half of its original strength. Half-lives are the easiest way for healthcare providers and scientists to easily calculate the length of time a medication may stay in a healthy human being.

Depending on your height and weight, an antibiotic's half-life may help determine how long it will stay in your body. Each antibiotic has a different half-life and can range from short half-lives of only an hour to half-lives over 68 hours before half of the drug has been eliminated from the body.

For example, amoxicillin has a half-life of about one hour. This means it only takes an hour for the amoxicillin dose to be reduced by half in your body.

What Other Factors Affect How Long Antibiotics Stay in the Body?

Other factors crucial to the amount of time an antibiotic may stay in the body can include:

• Liver or kidney function

• Body weight

• Dose

• Duration of treatment

• Type of antibiotic

For this reason, the prescribed dosing for an adult versus a child might differ.

Depending on the specific medication given, it could last in the body as long as a few hours or up to a few days.

Different formulations are also available for antibiotics, which can affect absorption in the body. These include:

• By mouth: Tablets, capsules, chewable, or solution

• By injection: In the arm, buttocks, thigh, or the veins

• Topically: Creams or ointments

Still, the specific type of antibiotic and dose play the most important role in determining the amount of time the drug will stay in your system.

Types of Antibiotics

Antibiotics can be either bactericidal or bacteriostatic. Bactericidal antibiotics kill the bacteria, whereas bacteriostatic antibiotics prevent the growth of bacteria.

Examples of bacteriostatic antibiotics include the following:

• Tetracyclines: Vibramycin (doxycycline), Minocin (minocycline)

• Lincosamides: Cleocin (clindamycin)

• Macrolides: Zithromax (azithromycin), Biaxin XL (clarithromycin), Erythrocin (erythromycin)

Bactericidal antibiotics include:

• Aminoglycosides: Tobrex (tobramycin)

• Beta-lactams, such as penicillins, cephalosporins, and carbapenems: Amoxicillin, cefazolin, Merrem (meropenem)

• Fluoroquinolones: Cipro (ciprofloxacin), levofloxacin, Avelox (moxifloxacin)

Dosage and Administration

The dosage and administration of your antibiotic will vary depending on the specific illness or infection present in the body.

Common short-course antibiotic regimens can range anywhere from a few days to up to two weeks. Acute infections will likely only require oral or topical antibiotic forms. However, more severe infections may warrant intravenous (IV) antibiotics administered through the veins.

Your healthcare provider will determine your dose based on the type of infection and antibiotic. Dosing recommendations may also change for people with kidney problems and children.

Generally, large doses of antibiotics are not always the best form of treatment. The bacteria in our body may become resistant to these high doses of antibiotics, which later become too strong to treat with traditional antibiotics. Antibiotic resistance occurs when the germs or bacteria fight off the drugs designed to kill or suppress them.

The likelihood of antibiotic resistance has increased rapidly due to the poor use of antibiotics. The right dose and medication for the right bug or bacteria should always be discussed with your primary healthcare provider.

Summary

Antibiotics will always play a crucial role in treating infections. As with any drug, it will take some time to completely leave your body after taking it. The length of time antibiotics stay in your system will vary but usually last from a few hours to a few days.

Other factors, such as health status and type of antibiotic, can influence the drug's clearance rate.

Always finish your duration of therapy as directed by your healthcare provider, even if you feel better, before completing the treatment course. It is best to eliminate the bacteria causing your illness completely rather than partially, as this could cause antibiotic resistance, leading to a more difficult-to-treat illness.

Talk to your healthcare provider if you have questions about your infection and antibiotic treatment.

Frequently Asked Questions

Will my diet affect how long antibiotics stay in my system?

Generally, most antibiotics are safe to take on an empty stomach. While taking your dose with food can prevent stomach upset, most diets do not affect how long an antibiotic will stay in the body.

Consumption of dairy products (e.g., cheese, yogurt, milk) could reduce how well certain antibiotics work. For example, you should take tetracyclines three hours before having any dairy products. Before starting treatment, ask your healthcare provider if dairy consumption will affect your particular antibiotic.

Will taking other medications affect the half-life of antibiotics?

Some antibiotics can be broken down in the liver. If you take other medications that are broken down in the liver, the antibiotic's half-life could be affected.

Always tell your healthcare provider about all your medications before starting a new treatment. They can inform you if any interactions are possible.

The author would like to recognize and thank Jaycob Mathew Peña for contributing to this article.