## Most Common Adverse Effect of Fluoroquinolones **Key Point:** Gastrointestinal disturbances (nausea, vomiting, diarrhoea, abdominal discomfort) are the MOST FREQUENT adverse effects of fluoroquinolones, occurring in 3–17% of patients across clinical trials. ### Frequency Hierarchy of Fluoroquinolone Adverse Effects | Adverse Effect | Incidence | Mechanism | Clinical Significance | |---|---|---|---| | **GI disturbances** | 3–17% (most common) | Direct irritation of GI mucosa; disruption of normal flora | Usually mild, self-limiting; rarely requires discontinuation | | Tendinopathy / rupture | 0.14–0.4% | Inhibition of collagen cross-linking; fluoroquinolone accumulation in tendon | Serious but rare; risk ↑ with age >60, corticosteroids, renal impairment | | Photosensitivity | 1–3% (older agents) | Phototoxic reactions; more common with older FQs (e.g. lomefloxacin) | Newer agents (levofloxacin, moxifloxacin) have lower risk | | QT prolongation | <1% | Blockade of cardiac K^+^ channels (hERG); dose- and concentration-dependent | Risk ↑ with hypokalaemia, hypomagnesaemia, bradycardia, drug interactions | **High-Yield:** While tendinopathy is a serious and well-publicized adverse effect, it remains RARE. GI side effects are far more common in routine clinical practice and are the reason many patients discontinue fluoroquinolones. **Clinical Pearl:** GI disturbances with fluoroquinolones can be minimized by taking the drug with food (though absorption may be slightly reduced) or spacing doses away from antacids and iron supplements. **Warning:** Do not confuse "most common" with "most serious." Tendon rupture is serious but rare; GI upset is common but usually benign.
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