It cannot reach adequate concentrations in the central nervous system to treat infections there, though. If this side effect occurs, it is most easily managed by giving the medication with food. Drugs of the Tetracycline class have potential to permanently stain teeth if given to immature animals. Doxycycline has the least potential for doing this. If these medications are used together, neither may be absorbed properly and the benefits of both are lost.
Iron containing vitamin supplements produce the same problem.
The reconstituted oral suspension is only good for 2 weeks. After that, it must be discarded. Doxycycline does not kill bacteria, it merely curtails their ability to reproduce. For the invading bacteria to be killed, the host's immune system must be active and effective. This may not be the best choice medication for immune compromised patients.