I searched for an answer on several different levels and here's what I found:
- FDA's presrcibing information for CIPRO(R) and for DORYX(R). Neither document mentions an interaction between these two drugs, nor for their classes.
These sources should be considered first as they are medicolegally binding and should include clinically relevant information.
- Interactions checkers such as rxlist and DrugBank. Rxlist's interaction checker returned nothing, and DrugBank's checker says that: "Doxycycline may decrease the excretion rate of Ciprofloxacin which could result in a higher serum level." This means that taking them together MAY increase ciprofloxacin effect through higher serum levels, but it doesn't say there how much higher and if it is clinically relevant.
I am not sure how reliable those databases are, because good interactions checkers are usually not publicly and freely available, since much time and energy are put into them, making them a commercial product that requires subscription or some other form of paid use.
- I found this paper titled "Interactions of the 4-quinolones with other
antibacterials". Doxycycline was not used in this study, but tetracycline (which belongs to the same class) was, and according to the results, tetracycline at sub-inhibitory concentrations did antagonize the activity of ciprofloxacin (p. 223).
Note, however, that this was an in-vitro study.
Taking all this information into account, I would conclude that no CLINICAL antagonistic interaction exists, i.e. it is possible to take them together if the treating doctor believes the combination has a therapeutic advantage. There might be some antagonism still, but again, this evidence is from an in-vitro study, and it is unknown if it is clinically relevant.
