How Long Do Antibiotics Take to Work?

Testing positive for strep throat or having symptoms of a urinary tract infection (UTI) can feel overwhelming. Thankfully, antibiotics can help fight off these and other bacterial infections. They can also prevent complications of an infection down the road.

As you pick up your antibiotic from the pharmacy, certain questions may be top of mind, like: How long does it take for antibiotics to work? And when will I start feeling better? We’ll answer these questions and more so you can focus on your recovery.

How long does it take for antibiotics to work?

How long an antibiotic takes to work — or its “onset time” — depends on a few factors. The most relevant ones are the antibiotic you’re taking and the infection being treated.

Antibiotics get to work shortly after you take your first dose. But it can take 24 to 72 hours (1 to 3 days) to start feeling better. Below, we’ll break down expected onset times by antibiotic class. We’ll focus on oral antibiotics that are commonly dispensed at retail pharmacies.

How long does it take to see full effects from antibiotics?

It typically takes 1 to 3 days to begin feeling better after starting an antibiotic. Symptom improvement is a sign that your antibiotic has set in and is working as it should. But you may not see the full benefits until you’ve completed your entire antibiotic prescription. You may also have to wait for your body’s natural inflammatory response to calm. Until this happens, you may continue to feel under the weather.

Even if you start feeling better after a couple days, make sure to finish your antibiotic course. Otherwise, some bacteria may linger in your body and contribute to antibiotic resistance. This means the antibiotic may not work now or in the future to treat your infection.

Good to know: If you’re not feeling better, you develop a fever, or your symptoms worsen after a few days taking your antibiotic, reach out to your prescriber. You may need a different antibiotic. Another possibility is you have a virus, which means an antibiotic won’t help.

Are there factors that can affect how long antibiotics take to work?

Your antibiotic and the type of infection you have are some of the top factors that impact how long your treatment takes to work. Drug interactions can also play a role. 

For instance, calcium supplements can bind to certain antibiotics — such as tetracyclines and fluoroquinolones. This can affect how much of your antibiotic is absorbed, potentially making it less effective. Separating calcium from tetracycline and fluoroquinolone antibiotics by about 2 hours helps prevent this interaction from happening.

Food can also occasionally affect how antibiotics are absorbed. Food can either delay their absorption or make them more effective, depending on the medication. Ask your pharmacist if you should take your antibiotic with or without food.

It’s a good idea to keep a current list of your medications and supplements handy. Your pharmacist and prescriber can screen for any interactions before you start an antibiotic.

What to expect when starting an antibiotic

Like all medications, antibiotics can cause side effects to be aware of. Common antibiotic side effects include:

• Nausea

• Vomiting

• Diarrhea

• Headache

• Dizziness

• Rash

Certain antibiotics, such as Augmentin, are known for causing stomach-related side effects. Taking your antibiotic with food may help ease an upset stomach. These symptoms should go away after you finish your antibiotic.

Diarrhea can happen within a few hours after your first antibiotic dose. But in some cases, you can develop diarrhea up to 2 months after you finish the prescription. This is a sign of a Clostridioides difficile (C. diff) infection related to antibiotic use, and it can be severe in rare cases. Seek medical attention right away if you experience serious symptoms such as severe diarrhea, stomach pain, and fever.

Also, watch for any signs of an allergic reaction when you start an antibiotic. Hives are one of the telltale signs. Make sure to inform your prescriber if you develop them. They may want to stop your current antibiotic and prescribe an alternative one. 

Anaphylaxis symptoms are a risk that can be life-threatening. These include trouble breathing or swelling of the face, throat, or tongue. In this situation, it’s important to seek emergency care immediately.

How long do you need to take antibiotics?

For most bacterial infections, you take antibiotics for 7 to 14 days. But the time frame may vary. 

For instance, you may be told to take Bactrim for 3 days to treat a UTI. Or you may receive a 5-day prescription for a UTI antibiotic called Macrobid (nitrofurantoin). 

On the other hand, you may need to take an antibiotic for longer than 14 days. For example, if you develop complications related to Lyme disease from a tick bite, you might need to take an antibiotic like doxycycline or amoxicillin for up to 28 days (4 weeks).

Frequently asked questions

Why trust our experts?

Written by:

Jennifer Gershman, PharmD, CPh, PACS

Jennifer Gershman, PharmD, is a licensed pharmacist in Florida. She has worked in the pharmacy industry for more than 15 years and is currently a medical writer.

Edited by:

Karla Robinson, MD

Karla Robinson, MD, is a medical editor for GoodRx. She is a licensed, board-certified family physician with almost 20 years of experience in health through varied clinical, administrative, and educational roles.

Our editorial standards

Meet our experts