If you are planning to administer the medication with food, remember not to give it with any food that contains iron or diary. The reason being food containing iron or calcium may block the medicine and thereby tamper its efficacy. Hence to help ease the stomach action and avoid any coincidental upsets of your pet’s stomach, avoid the administration of the medication with such foods. Doxycycline is an antibiotic that starts to exhibit its actions very quickly, say about 1-2 hours and you may see the effects within a few days.