Adverse Effects of Fluoroquinolones

Introduction to Neurologic Adverse Events

Neurologic adverse events occur in approximately 3% of patients taking fluoroquinolones 1

Patients with alcohol use disorder, malnourished individuals, patients taking medications that antagonize vitamin B1, patients with prolonged vomiting or poor food intake, pregnant women with hyperemesis gravidarum are at risk of thiamine deficiency-related peripheral neuropathy 2
Patients with certain medical conditions, such as alcoholics, renal dialysis patients, elderly individuals, post-operative patients, patients with infections or critical illness, pregnant women, and patients on certain medications, are at risk for vitamin B6 deficiency 3

The following populations are at risk for vitamin B6 deficiency and require monitoring and supplementation:

Population	Recommended Daily Dose
Adults and children >18 years	1.3-1.7 mg/day
Pregnant women	up to 2 mg/day
Premature and full-term infants up to 12 months	0.15-0.2 mg/kg/day
Older children	1.0 mg/day
Parenteral nutrition	4-6 mg per day

Patients with pernicious anemia, strict vegetarians/vegans, elderly individuals, patients with malabsorption syndromes, and patients with inflammatory bowel disease are at risk for vitamin B12 deficiency 2

When evaluating peripheral neuropathy, always rule out vitamin deficiencies (B1, B6, B12, folate) before attributing to other causes, with consideration of nerve conduction studies to confirm axonal neuropathy 2
Plasma pyridoxal 5-phosphate (PLP) is the preferred measurement for diagnosing B6 deficiency, with normal values ranging from 5-50 μg/L (20-200 nmol/L) 3
Inflammatory conditions can cause falsely low plasma PLP levels, making red blood cell PLP measurements more reliable in these cases 3
Low albumin and altered alkaline phosphatase activity can affect PLP concentration, requiring careful interpretation of laboratory results 3

For confirmed thiamine deficiency neuropathy, administer thiamine 50-100 mg daily for acute treatment, followed by maintenance doses of 25-50 mg daily, with response typically beginning within 24 hours 4
For diabetic peripheral neuropathy, consider a trial of thiamine 25 mg/day combined with pyridoxine 50 mg/day, with symptoms improving in 88.9%, 82.5%, and 89.7% of patients with pain, numbness, and paresthesia respectively 4
For acute vitamin B6 deficiency, 5g IV (1g for each gram of isoniazid ingested) is recommended, with administration of 1g IV/IM every 30 minutes up to a maximum of 5g 3
For deficiency from chronic poor intake, oral supplements of 50-100mg daily for 1-2 weeks are suggested, with PLP levels responding to intake and reflecting liver stores 3
The American College of Gastroenterology recommends vitamin B6 as a first-line treatment for nausea and vomiting of pregnancy (NVP) at doses of 10-25 mg every 8 hours, with a strength of evidence based on clinical guidelines 5

Patients on medications known to antagonize vitamin B1 should receive prophylactic supplementation, although the specific dosage is not provided in the article 2
Patients on isoniazid should receive prophylactic B6 supplementation, and patients undergoing continuous renal replacement therapy require careful monitoring and supplementation 3
Vitamin B12 and folate levels should be checked due to functional interdependence with vitamin B6, as B12 deficiency can be masked by high B6 levels and folate has considerable functional overlap with B6 and B12 6
Homocysteine levels should be checked, as elevated homocysteine may indicate functional B-vitamin deficiencies despite high B6 6
The National Kidney Foundation recommends supplementation with folate, B2, B6, and B12 for patients with hyperhomocysteinemia 7

Substantial functional recovery usually occurs within 3-6 months of initiating thiamine supplementation, with motor recovery typically better than sensory recovery 4
Vitamin B6 deficiency can cause rapid neurological deterioration, including seizures, which respond dramatically to supplementation, emphasizing the importance of prompt treatment 3
Excessive vitamin B6 supplementation (>500 mg/day) can cause sensory neuropathy 3