Possible Harmful Interaction
The herb fennel appears to reduce blood levels of ciprofloxacin, possibly impairing its effectiveness.
This finding comes from a placebo-controlled study in rats. Fennel might be expected to interfere similarly with other fluoroquinolone antibiotics.
Allowing 2 hours between taking ciprofloxacin and fennel should reduce the potential for an interaction, but may not eliminate it. For this reason, it may be advisable to avoid fennel supplementation during therapy with ciprofloxacin or other antibiotics in this family.