The Side Effects of Antibiotics
- , by SANUSq Research team
- 9 min reading time
Antibiotics are among the greatest achievements in medicine — genuinely life-saving — but they're not without downsides, and using them wisely matters more than ever.
Powerful, life-saving — and worth respecting
Let's be clear from the outset: antibiotics save lives. Infections that were once frequently fatal became treatable thanks to them, and when you genuinely need one, an antibiotic can be the difference between a minor illness and a dangerous one. Nothing in this article is a reason to refuse or stop a prescribed antibiotic. But precisely because they're so valuable, it's worth understanding antibiotics side effects and the bigger issue of resistance — not to fear them, but to use them wisely, so they keep working when we truly need them. A crucial first point is that antibiotics only work against bacterial infections; they do nothing for viral illnesses like colds and flu, which is why doctors rightly decline to prescribe them for those.
The common side effects
Like all effective medicines, antibiotics can cause side effects. The most common are digestive: nausea, diarrhoea, stomach cramps and loss of appetite, because antibiotics don't only target the bacteria causing your infection — they also disrupt the vast community of beneficial bacteria in your gut. Some people experience thrush (a fungal overgrowth) after a course, for the same reason. Allergic reactions can occur, ranging from mild rashes to, rarely, serious reactions that need urgent care. And certain antibiotics carry their own specific cautions, such as increased sun sensitivity or interactions with other medicines. Most side effects are mild and settle once the course finishes, but they're a reminder that these are powerful drugs, not sweets. If a side effect is troubling you, the right move is to speak to your prescriber or pharmacist rather than simply abandoning the course — they can advise on managing it, or switch you to something that suits you better.
The disruptive effect of antibiotics on the gut microbiota is well established. A review of the evidence found that some clinical data suggest probiotics can reduce antibiotic-related side effects, including Clostridioides difficile-associated diarrhoea, but concluded that no data causally link those clinical benefits to protection or recovery of the microbiota itself (Szajewska et al., 2025).
The gut microbiome takes a hit
One of the most significant, if invisible, effects of antibiotics is on your gut microbiome — the trillions of beneficial microbes that support digestion, immunity and much else. A course of antibiotics can sharply reduce the diversity of this community, and while it usually recovers, research shows the disruption can last for months, and sometimes the microbiome doesn't return exactly to its previous state. This matters because a healthy, diverse gut microbiome is linked to so many aspects of wellbeing. It's why supporting gut recovery after a course — through fibre-rich, varied whole foods and fermented foods — is a sensible idea, as we discuss in how to restore healthy gut flora. Probiotics can help reduce antibiotic-associated diarrhoea, though the evidence that they fully "restore" the microbiome is mixed and strain-dependent, so they're worth trying for that specific purpose without over-promising.
The bigger picture: antibiotic resistance
Beyond individual side effects lies a far larger concern — one of the biggest threats facing global health. Antibiotic resistance happens when bacteria evolve to survive the drugs designed to kill them, and the more antibiotics are used (and misused), the faster resistant strains emerge and spread.
Antibiotic resistance is a major public-health threat, and epidemiological studies show a direct relationship between antibiotic consumption and the emergence and spread of resistant bacterial strains (Pilmis et al., 2020).
The consequence is sobering: infections that are currently routine to treat could, in a world of widespread resistance, become dangerous again. This is why antibiotic "stewardship" — using these drugs only when genuinely needed — is everyone's business, not just doctors'.
When antibiotics are truly needed — and when they're not
Getting the most from antibiotics, with the least harm, comes down to using them for the right things. They are the correct and often essential treatment for genuine bacterial infections — conditions such as bacterial pneumonia, urinary tract infections, strep throat, and many wound and skin infections — and in serious cases they are unquestionably life-saving. Where they add nothing is against viruses, which cause the majority of coughs, colds, sore throats, flu and most cases of acute bronchitis and sinusitis. This is the crux of sensible use: the illnesses people most often want antibiotics for are frequently the very ones antibiotics can't touch. Taking them anyway means accepting the side effects and contributing to resistance while getting no benefit in return. It can be genuinely frustrating to be sent home from the doctor without a prescription when you feel wretched, but for a viral illness that advice is protecting both you and everyone else. The right response to most colds and flu is rest, fluids and time — and seeing a doctor if things are unusually severe, prolonged, or take a turn for the worse, since a secondary bacterial infection can occasionally develop and genuinely need treatment.
Using antibiotics wisely
The practical guidance here is simple and important, and it holds two ideas together at once: take antibiotics when you truly need them, and don't when you don't. Only take antibiotics prescribed for you, for a genuine bacterial infection — never pressure a doctor for them for a cold or flu, which they won't help. When you are prescribed a course, take it exactly as directed and complete it as advised by your prescriber, rather than stopping early because you feel better or saving leftovers for next time. Never share antibiotics or use someone else's. And support your body around a course with good hydration, rest, fibre-rich and fermented foods for your gut, and sensible everyday hygiene to avoid infections in the first place. Used this way — respectfully and only when needed — antibiotics remain the life-savers they were meant to be. If you ever have a severe reaction to an antibiotic, such as difficulty breathing or swelling, seek urgent medical help.
Frequently asked questions
What are the common antibiotics side effects?
The most common are digestive — nausea, diarrhoea, cramps and reduced appetite — because antibiotics disrupt beneficial gut bacteria as well as harmful ones. Thrush and allergic reactions can also occur. Most side effects are mild and settle once the course finishes, and none of them is a reason to stop a prescribed course without talking to your prescriber first.
Do antibiotics damage your gut?
Antibiotics reduce the diversity of the gut microbiome, and while it usually recovers, the disruption can last months and may not fully return to its previous state. Supporting recovery with fibre-rich whole foods and fermented foods is a sensible step.
Should I take probiotics with antibiotics?
Specific probiotics can reduce the risk of antibiotic-associated diarrhoea, and no clinical harms have been linked to taking them after antibiotics. Evidence that they fully restore the microbiome is mixed, so they're worth trying for that specific purpose without over-promising.
What is antibiotic resistance?
It's when bacteria evolve to survive the drugs meant to kill them. The more antibiotics are used and misused, the faster resistant strains emerge and spread — risking a future where currently routine infections become dangerous again. It's one of the biggest global health threats.
Should I stop antibiotics when I feel better?
Take the course exactly as your prescriber directs rather than stopping early just because symptoms improve, and don't save leftovers or share them. Follow your doctor's or pharmacist's specific advice, as guidance on course length is evolving for some infections.
Why won't my doctor give me antibiotics for a cold?
Because antibiotics only work against bacteria, not viruses — and colds and flu are viral. Taking antibiotics you don't need brings side effects and fuels resistance without helping you recover. Declining them for viral illness is good medicine, not neglect.
References
- Szajewska H, Scott KP, de Meij T, et al. Antibiotic-perturbed microbiota and the role of probiotics. Nature Reviews Gastroenterology & Hepatology. 2025;22(3):155–172. doi:10.1038/s41575-024-01023-x
- Pilmis B, Le Monnier A, Zahar JR. Gut Microbiota, Antibiotic Therapy and Antimicrobial Resistance: A Narrative Review. Microorganisms. 2020;8(2):269. PMC7074698
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