Antibiotics are drugs that are used in the treatment and prevention of bacterial infections. They may either inhibit the growth of bacteria or kill them outright. Antibiotics vary in their mode of action, effectiveness, and how they are administered. They usually start working very quickly, but they differ in how long they stay in your body, depending on the type of antibiotic, how long you take it, and a few other factors. 

In this article, we will go over what antibiotics are used for, types of antibiotics, and how long different types stay in your system.

What Are Antibiotics? 

Technically, the term “antibiotic” only refers to substances that come from natural sources such as bacteria and molds — synthetic antibiotics designed in labs should really be called antibacterial or antimicrobial drugs. However, in everyday usage we think of all drugs that help fight bacteria as antibiotics, no matter how we get them. (Source)

Antibiotics are used to treat infections caused by bacteria, such as strep throat, bacterial pneumonia, ear infections, and urinary tract infections. It’s key to note that antibiotics are not effective against viral infections, which may be treated with a different class of drugs called antivirals. This is why antibiotics don’t work against the common cold, which is a viral rather than bacterial infection. (Source, Source)

Antibiotics stay in your system for anywhere from a few hours to a few days. The exact amount of time will depend not just on the drug and how long you take it, but also on individual factors such as your age and body mass.

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Types of Antibiotics

Antibiotics can be characterized and grouped based on their mechanism of action, their spectrum of action (which bacteria they work against), and how they are administered. 

Antibiotics: Mechanism of Action

Antibiotics are either bactericidal, meaning they kill bacteria, or bacteriostatic, meaning they keep bacteria from reproducing while the immune system clears them away. They do this by interfering with:

• building and maintenance of bacterial cell walls
• production of DNA and RNA
• production of proteins needed for cellular function

(Source, Source)

Antibiotics: Spectrum of Action

Antibiotics can also be classified by their spectrum of action, which is the range of different pathogens they act against.

• Broad-spectrum antibiotics are effective against a wide range of bacteria and are especially useful when the target pathogen hasn’t been identified.
• Narrow-spectrum antibiotics are used to treat infections caused by specific types of bacteria after they’ve been identified.

Because broad-spectrum antibiotics affect many different bacteria, they may kill off not just the ones causing infection but also members of your normal microbiome — the community of microbes that populate your gastrointestinal tract and skin and are beneficial to your health. Narrow-spectrum antibiotics are less likely to do this. (Source, Source)

Antibiotics: Method of Administration

Antibiotics come in various forms and can be administered in various ways, including: 

• oral administration: taken by mouth, including tablets, capsules, and liquids 
• intravenous administration: injected into a vein, allowing the antibiotic to go directly into the bloodstream
• topical administration: applied to the skin through creams, lotions, and ointments

(Source)

How Long Do Antibiotics Stay in Your System

Antibiotics stay in your system for anywhere from a few hours to a few days. The exact amount of time will depend not just on the drug and how long you take it, but also on individual factors such as your age and body mass.

Understanding Antibiotic Half-Life

Each antibiotic, like every other medication, has its own half-life. The half-life is the amount of time it takes for one-half of the drug to be cleared from your body. Generally, it takes between 5 and 7 half-lives for all of a drug to leave your body once you stop taking it.

So, for example: 

• penicillin: Used in the treatment of many bacterial infections, including those caused by Staphylococcus and Streptococcus bacteria, penicillin has a half-life of about 90 minutes, so it takes about 8 hours for penicillin to leave your system. (Source)
• doxycycline: Used in the treatment of respiratory infections, sexually transmitted infections, and skin infections, doxycycline has a half-life of up to 22 hours, so it takes about 5 days for doxycycline to leave your system. (Source)

ciprofloxacin: Used in the treatment of many serious bacterial infections where safer antibiotics are ineffective, ciprofloxacin has a half-life of about 4 hours, so it takes about 22 hours for ciprofloxacin to leave your system. (Source)

Most antibiotics can range from a few hours to a few days until they are completely out of our systems.

What Affects How Long Antibiotics Stay in Your System?

While we can estimate how long antibiotics are expected to stay in your system, how long they do stay varies from one person to another. Some factors to consider include: 

• duration and dosage of drug: Higher doses of antibiotics, taken for longer periods of time, may take longer to clear.   
• metabolic rate: A slower metabolism means it will take longer for the antibiotic to be excreted from your system. 
• age: Older people may take longer to completely metabolize and clear antibiotics.
• overall health: Poorer overall health may increase the time the antibiotic remains in your system. 
• body mass: The larger your body mass, the longer it will take for the antibiotic to leave your system. 

(Source)

The Bottom Line

There are a number of factors that affect how long it takes an antibiotic to clear your system after your last dose, including the antibiotic itself, how long you took it, how old you are, and your overall state of health. Generally, the process takes anywhere from a few hours to a few days. 

Antibiotics, especially broad-spectrum antibiotics, may disrupt your normal gut microbiome, but dietary interventions may help you reduce the intestinal symptoms and other side effects that often come with antibiotic use. Our Care Team at WellTheory can help you to understand and unlock the power of food as medicine.