Doxycycline is an oral prescription medication used to treat such conditions as acne, gonorrhea, and chlamydia, acne-like lesions, periodontitis, as well as anthrax after the patient has been exposed to it. It is unlikely to treat viral infections, such as cold and flu. If you are taking any of the drugs mentioned below tell your doctor about that before using doxycycline: cholesterol-lowering medications, colestipol, isotretinoin, tretinoin, antacids, products that contain bismuth subsalicylate, minerals (iron, zinc, calcium, magnesium), penicillin antibiotics, blood thinners, vitamin and mineral supplements. You can continue taking this medicine if you have some of the following mild side effects: trouble swallowing, mild nausea and vomiting, mouth sores, swollen tongue, stomach upset, vaginal itching or discharge. Stop using doxycycline and talk to your doctor immediately if you experience any of the following serious side effects of doxycycline: loss of appetite, severe headache, dizziness, urinating less than usual, blurred vision, body aches, flu symptoms, severe blistering and peeling, yellowing of the skin or eyes, easy bruising or bleeding, unusual weakness, fast heartbeat, nausea and vomiting.