Adverse Effects of Antibiotics: Users Be Aware

“Over 25% of people lie to get antibiotics—and their lies aren’t even that good,”

Beth Mole, Ars Technica, October 30, 2026

“[W]hen you take an unnecessary antibiotic, two major harms occur. The first is unnecessary side effects…The second is called antibiotic resistance.”

Dr. Zach Gruss, PharmD, BCIDP, Infectious Diseases/Antimicrobial Stewardship Specialist, Medical University of South Carolina

Antibiotics—medicines that kill or inhibit the growth of bacteria—are arguably the most misused class of drugs. Their inappropriate use drives antimicrobial resistance (AMR) and other unintended consequences including both usual and unusual side effects.

In today’s post, I provide a brief recap on antibiotics and the unintended consequences of their misuse. And, since I’ve heard that “sex sells,” I end with a recently reported case of an allergic reaction to the antibiotic amoxicillin that developed in woman after having sex with a man taking the antibiotic. So, either read on…or just skip to the closing paragraphs.

The good news: a genuine medical miracle. Since Alexander Fleming discovered penicillin—the first antibiotic—in 1928, other antibiotics have been developed and continue to be developed. Antibiotics have been considered miracle drugs. They rank among the most consequential life-saving discoveries in the history of medicine, having saved hundreds of millions of lives globally. In 2023, approximately $30.7 billion was spent on all antibiotics, oral and injectable, across 62 countries. (Nandi, A., et al., “Global Trends in Antibiotic Expenditure: Analysis of Pharmaceutical Sales Data , 2013-2023,” Journal of Infectious Diseases, November 14, 2025).

The bad news, part one: resistance. The miracle-drug image is an important factor in driving patient desire for antibiotics. People want these drugs so badly that they often bend the facts to get them. In a survey of 959 adults, 27.9% said they had misrepresented symptoms to their clinicians, 26% reported exaggerating symptoms they really had, and 16.7% reported inventing symptoms that didn’t exist. (Rudd, T., “Here’s How Many Patients Fudge the Facts to Get Antibiotic Scripts,” MedPage Today, November 2, 2026). 

Patient desire for antibiotics “no matter what” has led to their misuse or inappropriate use. And the inappropriate use of antibiotics drives AMR. (The AMR pandemic is frequently featured in these posts. See, for example, Peterson, P., “Putting the Brakes on Antimicrobial Resistance,” Germ Gems, October 21, 2026). The numbers are sobering: AMR causes over 1 million direct deaths and is associated with 5 million deaths globally each year. The very success of antibiotics has set up a threat to their future.

The bad news, part two: side effects. AMR is not the only reason to think twice before taking an antibiotic. Like all medicines, antibiotics can have unnecessary side effects, including allergic reactions, nausea, vomiting, diarrhea and organ system damage.

The Agency for Healthcare Research and Quality estimates that that gastrointestinal disturbances occur in approximately 5-35% of patients receiving antibiotics. Disturbance of the gut microbiome is often the underlying cause, and these side effects are usually mild and self-limited. The trouble is these same symptoms occur in patients who develop Clostridioides difficle  (C. diff) infection.

C. diff is a spore-forming bacterium that infects the colon. Its toxins cause diarrhea and, in severe cases, a destructive inflammation of the colon called colitis. The organism can live quietly in the gut, and something has to change before its causes trouble. Usually that something is an antibiotic.  (See, Peterson, P., “Why C diff Should Be a Household Name,” Germ Gems, September 22, 2019).

C. diff  infection can develop in the community or in the hospital. In 2023, the Centers for Disease Control and Prevention found that 47% of C. diff cases were hospital-acquired infections and 53% were community associated. (Fulminant C. diff infection occurs in approximately 1-3% of cases, but mortality is approximately 30-40%.) Regardless of where it develops, C. diff is one of the major reasons to avoid unnecessary antibiotics.

The unexpected: an allergy passed between sexual partners. In my active clinical years, I saw a number of patients whose varied symptoms were precipitated by medications, antibiotics included. That experience left me with a reflex: when you see a patient with symptoms you don’t understand, blame the medications first. It now appears that you also have to consider and blame the medication that patient’s sexual partner is taking.

It was recently reported that a woman with a known allergy to penicillin developed anaphylaxis, a serious allergic reaction, immediately after intercourse with her husband, who was taking amoxicillin to treat a sinus infection. (Jeffery, S., “She Had a Life-Threatening Reaction After Sex,” MedPage Today, November 2, 2026). The man took the drug six hours before the couple had sex (without a condom) and five minutes later the woman developed symptoms of anaphylaxis. As the woman said, “Everything started to swell. I couldn’t breathe…Obviously, nearly dying is quite a terrifying experience.” (Wong, S., “Antibiotics can cause allergic reactions in sexual partners,” New Scientist Daily, October 31, 2026). It appears that even an antibiotic taken by someone else can affect the ultimate patient.

The lesson: respect antibiotics. Antibiotics are at once among medicine’s greatest triumphs and among its most carelessly used tools. They have saved hundreds of millions of lives globally; when they are truly needed, nothing replaces them. Yet each unnecessary prescription adds to the pressure that produces AMR and exposes the patient to side effects that range from a few days of nausea and diarrhea to life-threatening C. diff infection and, rarely, anaphylaxis, even in the partner of the person taking the antibiotic.

The lesson is not to fear antibiotics but to respect them; take them when a clinician determines they are needed, and be willing to hear “you don’t need one.” Its good medicine in its own right.