Antibiotics and Supplements: Probiotics and More

Calcium, magnesium, iron and zinc bind ciprofloxacin, levofloxacin, doxycycline and similar antibiotics, so take them 2 to 8 hours apart, depending on the drug^{1, 2}. Probiotics are fine alongside antibiotics: across 63 trials they cut antibiotic diarrhea by 42%³.

The probiotic benefit is real but uneven. It shows up most in children and in people on high-risk courses, and large trials in older hospital patients found none^{4, 5}. Our probiotics for diarrhea prevention page grades that evidence in full.

The problem supplements are minerals: calcium, magnesium, iron, zinc and aluminum bind fluoroquinolones and tetracyclines in the gut, so less antibiotic reaches the blood^{6, 2}. Iron and antacids also block cefdinir, a cephalosporin often prescribed for children⁷. Here is how common supplements sort, based on the drug labels^{1, 8}:

• Space them out: calcium, magnesium, iron, zinc, multivitamins with minerals and antacids, when the antibiotic is ciprofloxacin or a tetracycline (doxycycline, minocycline).
• Levofloxacin and moxifloxacin: space magnesium and aluminum antacids, iron, zinc and multivitamins. Calcium matters less here: the moxifloxacin label found no significant effect from a calcium supplement, and the levofloxacin label doesn't list calcium^{2, 9}.
• Cefdinir (Omnicef): take it at least 2 hours before or after iron supplements, multivitamins with iron and magnesium or aluminum antacids. A 60 mg iron dose cut its absorption by 80%. Iron with cefdinir can also turn stools reddish, which is harmless⁷.
• Not with milk alone: ciprofloxacin swallowed with only milk, yogurt or calcium-fortified juice; a mixed meal that contains them is fine.
• Avoid at the same time: bismuth subsalicylate, the pink stomach remedy, because it lowers the absorption of tetracyclines.
• Fine alongside: probiotics, which trials gave during the antibiotic course (see below).
• Go easy on caffeine with ciprofloxacin: the drug slows caffeine clearance, so caffeine from pre-workouts, guarana or green tea extract can build up and hit harder¹.
• Outside these warnings: supplements with none of these minerals and no caffeine, such as a plain vitamin D or fish oil capsule. Check the Supplement Facts panel of anything else for calcium, magnesium, iron or zinc.

If you take a prescription mineral, such as iron for anemia or a phosphate binder for kidney disease, ask your pharmacist to set the times; the ciprofloxacin label names binders such as sevelamer and lanthanum too¹. If you are weighing herbal options instead of a prescription, our review of natural antibiotics covers that evidence.

The gap depends on the antibiotic, and the labels set it:

• Ciprofloxacin: at least 2 hours before or 6 hours after products containing calcium, iron, zinc or magnesium¹.
• Levofloxacin: at least 2 hours before or 2 hours after antacids, iron and multivitamins with zinc⁹.
• Moxifloxacin: at least 4 hours before or 8 hours after magnesium, aluminum, iron or zinc².
• Cefdinir: at least 2 hours before or after iron supplements, multivitamins with iron and magnesium or aluminum antacids⁷.
• Doxycycline and minocycline: the labels give no hours, only that antacids with aluminum, calcium or magnesium and iron preparations impair absorption^{8, 10}. NIH advises taking these antibiotics at least 2 hours before or 4 to 6 hours after a magnesium or zinc supplement^{11, 12}.

The effect is large. In 12 healthy men, ciprofloxacin taken with ferrous sulfate reached about a third of its usual blood exposure (5.4 vs 14.5 µg·h/mL)¹³. Peak levels fell below what many susceptible bacteria need, and a multivitamin with zinc cut exposure by about a fifth¹³.

Calcium works the same way. Calcium carbonate antacid tablets taken 5 minutes before ciprofloxacin cut its bioavailability to about 60% of normal¹⁴. Taken 2 hours before, calcium carbonate left absorption unchanged (relative bioavailability 0.98), which is why spacing works¹⁵.

NIH's calcium fact sheet gives a shorter gap, 2 hours either side of a calcium supplement for quinolone antibiotics¹⁶. Where the two differ, follow your antibiotic's own label, which for ciprofloxacin asks for 6 hours after¹.

Iron is the classic problem for tetracyclines: a 1970 study found ferrous sulfate seriously impaired the absorption of tetracycline, oxytetracycline, methacycline and doxycycline, even in small doses¹⁷. A review put the share of fluoroquinolone users exposed to a chelation interaction at 22% to 76%, with magnesium-aluminum antacids worst, then iron, calcium and zinc⁶.

Calcium, dairy and doxycycline

Doxycycline has a low affinity for calcium, and its label says food or milk doesn't markedly change absorption⁸. Still, in 9 volunteers, 300 mL of milk lowered doxycycline absorption by 30% on average (range 9% to 53%)¹⁸. Minocycline, often used for acne, was absorbed normally with a high-fat meal that included dairy¹⁰.

Ciprofloxacin is more sensitive. Taken with 300 mL of milk or yogurt instead of water, its total absorption fell 30% to 36%, and peak levels fell 36% with milk and 47% with yogurt¹⁹. Our list of calcium-rich foods shows which other foods carry enough calcium to matter. For timing your other vitamins around meals, see when to take vitamins.

Supplements in this guide

• Calcium
• Magnesium
• Iron
• Zinc
• Multivitamin

For most people it isn't: pooled trials show fewer cases of diarrhea, though the benefit depends on who takes them and which antibiotic is used³. Probiotics are live microbes, mostly bacteria, meant to offset the beneficial gut bacteria an antibiotic kills along with the harmful ones²⁰.

• All ages, 63 trials, 11,811 people: probiotics lowered the risk of antibiotic-associated diarrhea by 42% (relative risk 0.58); 13 people had to take them to spare one case³.
• Children, 33 trials, 6,352 children: 8% got diarrhea with probiotics vs 19% without⁴.
• Outpatients, 17 trials, 3,631 people: 8.0% vs 17.7%²¹.
• C. difficile, 31 trials, 8,672 people: 1.5% vs 4.0%, with the benefit confined to trials whose baseline risk was above 5% (3.1% vs 11.6%)²².

The large trials in older hospital patients were null. PLACIDE gave 2,981 inpatients aged 65 and over a lactobacilli and bifidobacteria mix for 21 days: diarrhea struck 10.8% vs 10.4% on placebo⁵. A probiotic drink trial in 1,127 inpatients over 55 found 19.3% vs 17.9%²³.

NIH's fact sheet notes the benefit seen in children and adults aged 18 to 64 has not been shown in people 65 and over²⁰. Low-risk courses may not need a probiotic either: in a 2026 trial in healthy children given outpatient antibiotics for respiratory infections, 2% got diarrhea in each group²⁴.

Side effects were no more common with probiotics than without: 4% vs 6% in the children's trials, mostly gas, bloating, nausea and soft stools^{4, 22}.

Guidelines split for these reasons, which helps explain why doctors don't routinely prescribe probiotics with antibiotics. The American Gastroenterological Association suggests specific probiotics for adults and children on antibiotics to prevent C. difficile infection²⁵. The American College of Gastroenterology recommends against them for that purpose (conditional recommendation, moderate-quality evidence)²⁶.

The best-tested are Saccharomyces boulardii, a yeast, and Lactobacillus rhamnosus GG (LGG)^{27, 28}.

• S. boulardii: in 21 trials with 4,780 people, diarrhea fell from 18.7% to 8.5% (relative risk 0.47), in children and adults alike²⁷. NIH notes no clear dose-response across the doses tested²⁰. More on S. boulardii for diarrhea.
• LGG: in a review of 12 trials, pooled data from 11 trials (1,308 people) showed diarrhea falling from 22.4% to 12.3% (relative risk 0.49)²⁸. The benefit was clear in children; in adults it was significant only during H. pylori treatment (4 trials, relative risk 0.26)²⁸. See L. rhamnosus for diarrhea and probiotics during H. pylori treatment.
• Dose: in children, 5 billion CFU (colony-forming units) a day or more cut diarrhea from 23% to 8%; lower doses fell short of significance⁴.
• Named mixes: the AGA guideline lists S. boulardii and three specific combinations of Lactobacillus, Bifidobacterium and Streptococcus strains²⁵.

Yogurt and kefir are weaker bets. Two small yogurt trials (314 people) showed no consistent effect²⁹, and a probiotic yogurt trial in 314 inpatients with a mean age of 76 found 23.0% vs 17.6% on placebo yogurt³⁰. Kefir didn't prevent diarrhea in 125 children aged 1 to 5 (18% vs 21.9%)³¹. MedlinePlus still lists yogurt as an option that may lower the risk³².

Start on the first or second day of the antibiotic course and keep going for a few days after finishing it^{33, 32}.

• Start early: across 19 trials of hospitalized adults (6,261 people), probiotics cut C. difficile infection by 58% overall (relative risk 0.42). In the trials that started them within 2 days of the first antibiotic dose, the cut was 68% (relative risk 0.32); later starts showed no significant effect (relative risk 0.70)³³.
• Time of day: no trial has compared taking a bacterial probiotic with the antibiotic dose versus a few hours away. Keeping them a few hours apart costs nothing, but it is a precaution, not a tested rule.
• S. boulardii is a yeast: antibacterial drugs don't kill yeasts (the doxycycline label warns of fungal overgrowth for this reason)⁸. So S. boulardii is less likely to be affected by dose timing, though no trial has tested this.
• After the course: MedlinePlus advises continuing for a few days after the last dose³²; one trial continued for 5 days after³⁰.

Don't stretch it into months to restore your gut. In a study in mice and people, a multi-strain probiotic taken after antibiotics delayed the return of the gut's own bacteria compared with no treatment³⁴. It measured microbes, not symptoms, so it isn't proof of harm, but it is a reason to stop once the course and a few days after it are done. Our gut microbiome hub covers the wider evidence.

Broad-spectrum antibiotics cause the most trouble. The CDC lists fluoroquinolones, third- and fourth-generation cephalosporins, clindamycin and carbapenems as the highest-risk drugs for C. difficile³⁵. MedlinePlus names ampicillin, clindamycin, fluoroquinolones and cephalosporins³⁶.

• Penicillins: in a 2026 trial, children given broad-spectrum penicillins had diarrhea at 9.1%, against 0.51% of those on narrow-spectrum antibiotics²⁴. NIH also names penicillin and erythromycin among the drugs most often linked to it²⁰.
• Doxycycline: because it is almost completely absorbed, lower-bowel side effects such as diarrhea are infrequent⁸.
• Course length: longer courses may double the risk of C. difficile infection³⁷.

Who takes them matters too. Up to 30% of people on antibiotics get diarrhea, with higher rates in hospital patients, children under 2 and adults over 65²⁰. Children's episodes tend to start sooner and end sooner than adults'³⁸.

Most antibiotic diarrhea is mild and ends once the course stops, so the job is to stay hydrated and eat plainly until then^{8, 39}. Don't stop the antibiotic early on your own, since skipped doses make it less effective; call the prescriber instead⁸.

• Fluids: 8 to 10 glasses of clear fluids a day, plus at least 1 cup (240 mL) after each loose stool³⁹.
• Food: small meals; some salty foods; bananas and potatoes for potassium; white rice, white bread, pasta and oatmeal; peeled, cooked vegetables³⁹.
• Skip for now: fried and greasy food, gassy vegetables such as broccoli and beans, caffeine, alcohol and fizzy drinks; cut dairy if it makes things worse³⁹.
• Careful with bismuth: bismuth subsalicylate lowers the absorption of tetracyclines such as doxycycline⁸.

Probiotics shorten it a little: in children's trials, by about 0.9 days⁴. For persistent loose stools without antibiotics, see frequent diarrhea.

When is it C. diff?

Call your clinician for watery diarrhea that won't stop, bloody stools, stomach cramps or fever¹. Colitis from Clostridioides difficile typically brings watery diarrhea, often 5 to 10 times a day, sometimes with blood³⁶. It accounts for 15% to 25% of antibiotic-associated diarrhea³⁵.

It can start late. The risk is up to 10 times higher during the course and the month after it³⁷, and labels warn it can appear more than 2 months after the last dose¹. For ordinary diarrhea, MedlinePlus advises calling if an adult isn't better in 5 days, or a child in 2 days³⁹.

Diarrhea that lasts for weeks is a different problem. Zinc deficiency commonly causes diarrhea in children, and severe niacin deficiency (pellagra) can cause it too^{12, 40}. See zinc for diarrhea and vitamin B3.

Probiotics are generally safe for healthy people; the rare serious infections linked to them occur mostly in people who are severely ill or have a weakened immune system²⁰.

• Central lines, intensive care, feeding tubes: at least 60 reports of fungemia (yeast in the blood) have been linked to Saccharomyces probiotics, many in ICU patients or people with central venous catheters²⁰. A review of 92 invasive Saccharomyces infections found catheters and antibiotic therapy the most common risk factors⁴¹.
• Preterm infants: the FDA warned in 2023 that probiotics may cause invasive, potentially fatal infections in them²⁰.
• Weakened immunity: the Cochrane review judged short-term probiotics safe only for people who are not immunocompromised or severely debilitated²².
• Kidney disease: phosphate binders, including calcium acetate, also bind ciprofloxacin and need the same spacing¹.
• Older hospital patients: the largest trials found no benefit, so the case for a probiotic is weaker⁵.

Put minerals in the middle of the day, away from both antibiotic doses, and start the probiotic on day 1 or 2. Based on the gaps above^{1, 2, 7, 11}:

• Ciprofloxacin at 8 am and 8 pm: calcium, iron, zinc or a multivitamin at 2 pm, 6 hours after the morning dose and 6 hours before the evening one¹.
• Doxycycline at 8 am and 8 pm: magnesium or zinc at 2 pm, so the morning dose comes 6 hours before it and the evening dose 6 hours after^{11, 12}.
• Moxifloxacin once at 8 am: minerals with dinner at 6 pm, 10 hours after the dose and 14 hours before the next².
• Cefdinir at 8 am and 8 pm: iron, a multivitamin with iron or a magnesium or aluminum antacid at 2 pm, well over the 2 hours the label asks for on each side⁷.
• Probiotic: once a day with a meal from day 1 or 2, continued for a few days after the last antibiotic dose^{33, 32}.

If your doses fall at other times, give your pharmacist a list of everything you take, including antacids, since they carry the same minerals¹.

We searched PubMed for systematic reviews, meta-analyses and randomized trials of probiotics for antibiotic-associated diarrhea and C. difficile, and for absorption studies of antibiotics taken with minerals and dairy. We read 25 journal records and cited 24 of them, plus 17 web sources: current DailyMed labels for ciprofloxacin, levofloxacin, moxifloxacin, cefdinir, doxycycline and minocycline, NIH Office of Dietary Supplements fact sheets, CDC, MedlinePlus and the AGA guideline summary.

We gave meta-analyses and large trials the most weight and reported the null trials alongside the positive ones. The mineral studies are small crossover trials in healthy volunteers, and no trial has tested the best hour of the day for a probiotic. Researched October 14, 2026.

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2. DailyMed (US National Library of Medicine). Moxifloxacin hydrochloride tablets, film coated: prescribing information (American Health Packaging). https://dailymed.nlm.nih.gov/dailymed/services/v2/spls/1ed191f5-7df5-488c-bb72-91ac0b618d9a.xml
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8. DailyMed (US National Library of Medicine). Doxycycline hyclate tablets, USP: prescribing information (Alembic Pharmaceuticals Inc.). https://dailymed.nlm.nih.gov/dailymed/services/v2/spls/cff08863-7156-49d0-abbd-8f9274302886.xml
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10. DailyMed (US National Library of Medicine). Minocycline hydrochloride capsules: prescribing information (Actavis Pharma, Inc.). https://dailymed.nlm.nih.gov/dailymed/services/v2/spls/a5fc4d50-50b2-46e0-b722-4c6b2ec47d06.xml
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