Yes, you can absolutely get sick while taking antibiotics. Some of that sickness may come from the antibiotics themselves, some from new infections the antibiotics can’t touch, and some from complications triggered by the way antibiotics reshape your gut. About 1 in 5 people on antibiotics develops diarrhea during their course, and that’s just one of several ways you might feel worse before you feel better.
Antibiotics Themselves Can Make You Feel Sick
The most common complaint during an antibiotic course is gastrointestinal distress: nausea, vomiting, diarrhea, and stomach cramps. These aren’t signs that something has gone wrong. They’re a predictable consequence of how antibiotics work. To kill harmful bacteria, antibiotics also destroy helpful bacteria in your gut. That disruption to your digestive ecosystem is often enough to cause loose stools or nausea that lasts for days or even the full length of your prescription.
Skin rashes are actually the single most reported adverse reaction, affecting roughly 1 in 5 patients across clinical settings. Gastrointestinal symptoms follow at a lower rate in formal reporting, though many mild cases go unreported. These side effects are dose-related, meaning they tend to get worse at higher doses and ease up when the course ends. They are not allergic reactions, even though many people assume a rash or upset stomach means they’re “allergic” to the drug.
Viral Infections Don’t Care About Antibiotics
Antibiotics work by destroying bacterial cell membranes and blocking bacterial replication. Viruses aren’t cells. They’re smaller particles that hijack your own cells to reproduce, and they have no cell membranes for antibiotics to target. That means if you catch a cold, the flu, or a stomach virus while you’re on antibiotics, the medication will do nothing to stop it.
This is more common than people realize. Over 90% of sinus infections are caused by viruses, and bronchitis is almost always viral. If you’re taking antibiotics for a bacterial ear infection and then develop a sore throat and congestion, there’s a good chance you’ve picked up something viral. The antibiotic won’t prevent it, shorten it, or lessen the symptoms.
Yeast Infections and Other Secondary Infections
Antibiotics don’t just kill the bacteria causing your infection. They wipe out the normal bacterial communities that keep other organisms in check. When those protective bacteria are gone, yeast and fungi can grow unchecked, leading to secondary infections. Vaginal yeast infections are the classic example: they’re one of the most common complications of antibiotic therapy, especially with broad-spectrum drugs. Oral thrush, a yeast overgrowth in the mouth, is another possibility.
These secondary infections can start while you’re still mid-course, which creates the frustrating experience of treating one problem while a new one develops.
The Die-Off Reaction
For certain infections, particularly syphilis, Lyme disease, and leptospirosis, starting antibiotics can trigger a temporary inflammatory reaction known as a Jarisch-Herxheimer reaction. When the antibiotic rapidly kills large numbers of bacteria, the dying organisms release inflammatory substances into your bloodstream. The result feels a lot like the flu: fever, chills, headache, muscle aches, nausea, and general malaise.
This reaction typically begins within 2 hours of starting the antibiotic and resolves on its own within 12 to 24 hours. It’s not dangerous in most cases, but it can be alarming if you don’t expect it. The important thing to know is that it’s actually a sign the antibiotic is working, not failing.
When the Antibiotic Isn’t Working
Sometimes the sickness you feel while on antibiotics is the original infection refusing to clear. Antibiotic failure happens when the prescribed drug isn’t effective against the specific bacteria involved. This can occur because the bacteria are resistant to that antibiotic, because the bacteria have formed a protective layer called a biofilm that shields them from the drug, or because your immune system is compromised and can’t cooperate with the antibiotic to finish the job.
If your symptoms haven’t improved at all after two to three days on antibiotics, or if they’re actively getting worse, that’s a sign the treatment may need to change. Persistent or worsening fever, increasing pain, and new symptoms that weren’t present before starting treatment all warrant a call to your prescriber.
C. Diff: The Complication Worth Knowing About
The most serious gut-related complication of antibiotic use is an infection with Clostridioides difficile, commonly called C. diff. When antibiotics strip away your normal gut bacteria, C. diff can multiply rapidly and produce toxins that inflame your colon. Diarrhea and fever occur in almost all cases. In severe cases, the infection can progress to colitis, dangerous swelling of the colon, organ failure, or worse.
The red flags that distinguish C. diff from ordinary antibiotic-related diarrhea include watery diarrhea occurring three or more times a day, fever above 101.3°F (38.5°C), severe abdominal pain or tenderness, and diarrhea that contains blood or mucus. C. diff can develop during your antibiotic course or in the weeks after finishing it. If you develop these symptoms, don’t wait to see if they pass.
Allergic Reactions vs. Normal Side Effects
Stomach upset, nausea, and diarrhea are not signs of a true antibiotic allergy. They’re side effects related to the dose and mechanism of the drug. A true allergic reaction involves the immune system responding to the antibiotic as a threat. Immediate allergic reactions happen quickly, often within an hour, and can include hives, swelling of the face or throat, difficulty breathing, and in serious cases, anaphylaxis. About 2.5% of patients in clinical settings report anaphylaxis-level reactions.
Delayed allergic reactions can also occur days into a course, typically showing up as widespread rashes or skin peeling. These are immune-mediated and distinct from the common mild rash that some antibiotics cause. The practical distinction matters: a side effect means you may be able to take that antibiotic again with management, while a true allergy means you should avoid it in the future.
Reducing Side Effects During Your Course
Taking probiotics alongside antibiotics can meaningfully lower your risk of diarrhea. A large meta-analysis found that probiotics reduced the risk of antibiotic-associated diarrhea by about 39%, with the strongest evidence supporting Lactobacillus-based formulations. For every 11 patients who took probiotics with their antibiotics, one case of diarrhea was prevented. The protective effect was even stronger against C. diff, cutting the risk by roughly 63%.
Spacing your probiotic dose a few hours away from your antibiotic gives the beneficial bacteria a better chance of surviving. Eating with your antibiotic when the label allows it can also reduce nausea. Staying hydrated is especially important if you’re experiencing diarrhea, since the fluid loss can compound the fatigue and weakness you’re already feeling from the infection itself.