Other quinolone antibiotics such as ofloxacin, moxifloxacin, and levofloxacin and lomefloxacin do not appear to affect CYP1A2, and therefore do not interact with tizanidine.
Tizanidine has a narrow therapeutic range and a low oral bioavailability due to the extensive first-pass metabolism via cytochrome P450 (CYP 1A2). Concurrent use of tizanidine and strong CYP1A2 inhibitors, such as ciprofloxacin 100 mg/ml oral suspension, is not recommended because it may significantly increase tizanidine levels which can lead to severe adverse events.
Tizanidine is a substrate for cytochrome P450 1A2 (CYP1A2). [ciprofloxacin 100 mg/ml oral suspension] is a potent CYP1A2 inhibitor. Preferred management options are to use an alternative medication that is not a strong CYP1A2 inhibitor or to discontinue the tizanidine (see the "Alternative Options" box). It is also recommended to advise the patient to monitor for signs of hypotension, including: dizziness, fatigue, weakness, and somnolence.
Ciprofloxacin, fluvoxamine and zafirlukast inhibit CYP 1A2 and can increase the tizanidine area-under-the-concentration time curve (AUC) by more than 5-fold increased of tizanidine.7 Previous studies found that concurrent use tizanidine with ciprofloxacin or fluvoxamine increased tizanidine AUC levels by 10-fold and 33-fold, respectively.1, 5, 8
In a pharmacokinetic study, ciprofloxacin increased the area under the plasma-concentration curve of tizanidine by an average of 10-fold (range 6-fold to 24-fold).1, 6 Concurrent use of tizanidine and ciprofloxacin has been reported to induce adverse effects of tizanidine such as lowering blood pressure (BP), and heart rate (HR).9 A retrospective cohort study of 199 patients taking tizanidine plus ciprofloxacin and 960 patients taking tizanidine plus a different antibiotic found that co-administration of tizanidine and ciprofloxacin was associated with an increased risk of hospitalization by 2.19-fold (95% CI, 0.88-5.02) within 7 days after prescription of the antibiotic, 1.52-fold (95% CI, 0.63-3.33) within 14 days and 1.68-fold (95% CI, 0.84-3.17) within 30 days. 6
In healthy adults, co-administration of tizanidine and ciprofloxacin can decrease systolic blood pressure by 35 mm Hg.1 Furthermore, case reports have noted incidences of severe hypotension when tizanidine was administered concomitantly with ciprofloxacin.4, 9 Also, a retrospective study of ICSRs registered in the WHO global database until March 1, 2017 reported that among 91 patients receiving both tizanidine and ciprofloxacin concurrently, two fatal cases were reported.2
A previous study also reported that fluvoxamine increased tizanidine AUC up to 33- fold.5, 8 Concurrent use of tizanidine and fluvoxamine can reduce systolic blood pressure by 36 mm Hg.5, 8 Tizanidine adverse symptoms such as low heart rate, low body temperature, dry mouth, and enuresis has been reported in a patient taking tizanidine and fluvoxamine simultaneously.4 Moreover, a retrospective study of 913 patients treated with tizanidine found that among 23 patients who received fluvoxamine together with tizanidine, 6 patients experienced tizanidine adverse event such as low heart rate, dizziness, low body temperature, drowsiness, hypotension or speech disorder.4 Case reports have noted instances of dizziness when tizanidine was administered concomitantly with fluvoxamine.5, 10 Moreover, a cohort study of 1,626 patients prescribed tizanidine and 5,012 prescribed cyclobenzaprine concurrently with a strong CYP1A2 inhibitor (either ciprofloxacin or fluvoxamine), found tizanidine cohort had higher risk of experiencing of hypotension (OR=1.26, 95% CI, 1.03-1.54) and severe hypotension, SBP ≤70 mm Hg, with an OR= OR=1.60, 95% CI, 1.05–2.45) compared to cyclobenzaprine cohort.5