Ciprofloxacin

Is ciprofloxacin safe?

For a serious infection that needs it, yes, and for most people a course causes nothing worse than an upset stomach. In trials of 49,038 people, the most common side effects were nausea (2.5%) and diarrhoea (1.6%). The problem is a rare set of reactions that can be disabling and permanent: torn tendons, nerve damage and changes in mood and thinking, sometimes all in the same person. They carry a US boxed warningBoxed warningThe strongest safety warning the US FDA puts on a medicine's label, printed inside a black border at the top.More in the glossary, and regulators have spent 15 years narrowing when ciprofloxacin should be used.

We score it 64. That is well below amoxicillin (91), the antibiotic most people get first, because ciprofloxacin's rare harms are worse and its interaction list is much longer. The score does not mean a course already prescribed should be stopped without advice; talk to your doctor or pharmacist.

Why it has a boxed warning

Ciprofloxacin is a fluoroquinolone, the same family as levofloxacin and moxifloxacin. The US warning started with tendons in 2008 and was widened in 2016 to "disabling and potentially irreversible" reactions. The current Cipro label lists:

• Tendinitis and tendon rupture, most often the Achilles, sometimes both legs. It can start within hours or months after the course ends. Risk is higher over 60, with steroids such as prednisone, and after an organ transplant, but it also happens in young, healthy people.
• Peripheral neuropathy: burning, tingling, numbness or weakness in the hands and feet, which may be permanent.
• Central nervous system effects: anxiety, insomnia, confusion, hallucinations, depression and, rarely, suicidal thoughts. The MHRA highlighted the suicide risk again in 2023.
• Worse muscle weakness in myasthenia gravis, which can affect breathing.

Other label warnings include aortic aneurysm and dissection, mainly in older people (the FDA added this in December 2018), severe low blood sugar leading to coma or death in people on diabetes drugs, liver injuryDrug-induced liver injuryLiver damage caused by a medicine, herbal product or supplement. It is usually rare and unpredictable, but it can be serious.More in the glossary, QT prolongationQT prolongationA delay in the heart's electrical recovery between beats, seen on an ECG. Some medicines cause it, and in rare cases it leads to a serious heart rhythm problem.More in the glossary, sun sensitivity and C. difficile diarrhoea. In 2020 the MHRA also flagged a small increase in heart valve leakage.

When should ciprofloxacin be used?

Less often than it used to be. The US label tells doctors to use it for flare-ups of chronic bronchitis, simple bladder infections and sinusitis only when there is no other option. The UK went further in January 2024: all systemic fluoroquinolones should be prescribed only when commonly recommended antibiotics are inappropriate, because of resistance, allergy, side effects or treatment failure.

It still has a real role in kidney infections, some prostate, bone and gut infections, typhoid, anthrax and plague exposure, and infections resistant to other antibiotics. Children get it only for a few indications; in one study musculoskeletal problems were seen in 9.3% of children on ciprofloxacin vs 6% on other antibiotics.

Doses on the label

Infection (adults, Cipro label)	Dose	Usual length
Uncomplicated cystitis	250 mg every 12 hours	3 days
Urinary tract infection	250 to 500 mg every 12 hours	7 to 14 days
Skin, lower respiratory	500 to 750 mg every 12 hours	7 to 14 days
Bone and joint	500 to 750 mg every 12 hours	4 to 8 weeks
Chronic bacterial prostatitis	500 mg every 12 hours	28 days

People with reduced kidney function take lower or less frequent doses. Drink enough fluid during the course.

The interactions that matter

Ciprofloxacin blocks the liver enzyme CYP1A2CYP450 enzymesA family of liver enzymes that break down many medicines. When another drug or supplement speeds them up or slows them down, drug levels change.More in the glossary, so several drugs build up:

• Tizanidine (a muscle relaxant): contraindicatedContraindicationA situation in which a medicine should not be used, because the risk clearly outweighs any benefit, such as a known allergy to it.More in the glossary, because of dangerous drops in blood pressure and sedation.
• Theophylline: serious and fatal reactions, including seizures and cardiac arrest.
• Duloxetine levels rise about fivefold; clozapine, olanzapine, ropinirole and caffeine also rise.
• Warfarin: stronger anticoagulant effect, so INR needs checking.
• Diabetes tablets and insulin: risk of severe low blood sugar.
• Drugs that prolong the QT intervalQT prolongationA delay in the heart's electrical recovery between beats, seen on an ECG. Some medicines cause it, and in rare cases it leads to a serious heart rhythm problem.More in the glossary, such as some antiarrhythmics, antipsychotics and macrolide antibiotics.

Absorption is the other trap. Take ciprofloxacin at least 2 hours before or 6 hours after antacids, sevelamer, sucralfate, or supplements containing calcium, iron, zinc or magnesium. Do not swallow it with milk or yoghurt on their own.

Who should be careful

Ask your prescriber about alternatives if you are over 60, take steroid tablets, have had a transplant, have a known aortic aneurysm or heart valve disease, have myasthenia gravis, epilepsy or a history of depression or psychosis, or have reacted badly to a quinolone before. Stop and seek medical advice at the first sign of tendon pain, tingling, muscle weakness or a change in mood. Diarrhoea that is severe or bloody, even weeks later, needs checking for C. difficile; the evidence on probiotics for prevention is mixed.

How we scored it

Dependence (95), track record (90) and overdose marginOverdose marginHow far a normal dose is from a harmful one. A wide margin means an accidental extra dose is unlikely to cause serious harm.More in the glossary (80) are strong. Regulatory warnings (20) is the weakest factor: a boxed warningBoxed warningThe strongest safety warning the US FDA puts on a medicine's label, printed inside a black border at the top.More in the glossary plus formal restrictions in the US and UK. Serious side effects (50) and interactions (50) sit in the middle, reflecting rare but disabling reactions and a long list of drug and supplement clashes.