Antibiotics are medications that fight off bacterial infections, including urinary tract infections (UTIs), strep throat, and more.
But despite their ability to treat potentially dangerous infections, many people can experience uncomfortable side effects when taking antibiotics, including bloating, indigestion, nausea, and vomiting. Though less common, some antibiotics can also make you feel tired or weak.
In this article, I’ll describe how antibiotics work, the different groups of antibiotics, and what they’re used to treat. I’ll also cover which side effects are most commonly seen with antibiotic use—as well as some of the less common side effects.
Finally, I’ll cover when you may want to speak with a doctor for more personalized care.
Antibiotics are a class of powerful medications that eradicate bacteria or slow their growth, which can cure infections caused by bacteria.
Antibiotics should only be used to treat certain bacterial infections, and they are not effective against viral infections.
How Antibiotics Work
Antibiotics work by destroying bacteria or making it difficult for them to grow and reproduce in humans or animals.
They can be taken in a variety of forms, including:
- Orally (via liquids, tablets, sprays, drops, or capsules)
- Topically (through the skin by way of a cream or ointment)
- Via injection (intravenously or intramuscular)
Types of Antibiotics
Antibiotics are often classified into groups.
Here are some of the most common:
- Penicillins and penicillin-based: These are among the oldest antibiotics, and are a first-line treatment for many conditions, including respiratory tract infections. Amoxicillin is an example of a penicillin-based antibiotic.
- Tetracyclines: Often prescribed for common conditions like acne, skin infections, tick-borne illnesses, respiratory infections, and more.
- Cephalosporins: These antibiotics treat a wide range of infections, including ear infections, pneumonia, and meningitis.
- Macrolides: A common alternative for people who are allergic to penicillin, these antibiotics are used to treat some types of pneumonia, STDs, and other infections. Clindamycin and azithromycin are examples of macrolide antibiotics.
- Fluoroquinolones: These versatile antibiotics, including ciprofloxacin and levofloxacin, are used to treat a variety of skin, sinus, joint, and urinary infections. However, fluoroquinolones can interact with many common medications and may have some serious side effects.
- Sulfonamides: The most commonly prescribed sulfonamide is trimethoprim-sulfamethoxazole, sold under the name Bactrim or Septra. Sulfonamides work by stopping or slowing the growth of bacteria, and are often used for UTIs and skin infections.
Uses for Antibiotics
Your doctor can prescribe oral, topical, or intravenous antibiotics for a diverse range of bacterial infections, including:
- E. coli
- Skin infections
- Strep throat
- Tooth infection
- Urinary tract infections
- Whooping cough
In some cases, antibiotics can be life-saving, especially if you’ve been diagnosed with a serious illness like bacterial meningitis or sepsis, or if you’re at a higher risk of bacterial infection, including if you’re immunocompromised or receiving chemotherapy. Antibiotics are not used to treat viral infections.
Common Antibiotics Side Effects
Like many medications, antibiotics can cause unwanted side effects.
The most common side effects of antibiotic use include:
These side effects are generally mild, and resolve after you complete your course of treatment. Less common side effects are also possible. One of the rare side effects of antibiotic use is fatigue, or a general feeling of tiredness or weakness.
Specifically, these three antibiotics may cause you to feel tired:
Some antibiotics can also cause an allergic reaction. Before taking a new medication, talk to your doctor about any known allergies.
What to Do if Antibiotics Make You Sleepy or Tired
If you think an antibiotic may be causing you to feel sleepy or tired, talk to your provider. They can help switch you to another medication that may not cause this unwanted side effect.
Keep in mind that many of the conditions that antibiotics are used to treat can also cause fatigue, so your provider may ask additional questions to help determine whether or not your fatigue is a side effect of the medication, or the condition itself.
In the meantime, take the necessary precautions to stay safe. Don’t drive or engage in potentially harmful activities while feeling tired. Avoid taking additional substances that could increase your tiredness.
When to See a Doctor
If you’re experiencing any uncomfortable side effects while taking antibiotic medication, talk to your provider about whether or not another prescription may be right for you. The sooner you receive the appropriate treatment, the sooner you can start to feel better.
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Frequently Asked Questions
K Health articles are all written and reviewed by MDs, PhDs, NPs, or PharmDs and are for informational purposes only. This information does not constitute and should not be relied on for professional medical advice. Always talk to your doctor about the risks and benefits of any treatment.
K Health has strict sourcing guidelines and relies on peer-reviewed studies, academic research institutions, and medical associations. We avoid using tertiary references.
The relationship between short-term antibiotic treatments and fatigue in healthy individuals. (1993).
Be Antibiotics Aware: Smart Use, Best Care. (2020).
A patient-centred web-based adverse drug reaction reporting system identifies not yet labelled potential safety issues. (2021).