Doxycycline is a medicine that you can be prescribed if you have pneumonia, some other respiratory infection, acne, Lyme disease or some other bacterial infections it may be used to treat. You will need to use a reliable birth control method while using doxycycline to avoid getting pregnant, as this drug can cause harm to an unborn child. It may cause permanent teeth discoloration in unborn child. If doxycycline is taken by a breastfeeding woman, it can affect bone and tooth development of the infant. Tell your health care provider if you get severe headache, red skin rash, urinating less than usual, blurred vision, peeling, dizziness, confusion or weakness, body aches, severe pain in your upper stomach, pale or yellowed skin, flu symptoms, loss of appetite, yellowing of the skin or eyes, severe blistering, or easy bruising or bleeding, as those are serious side effects of doxycycline that can mean something is going wrong in your treatment. The following side effects are a lot more common when doxycycline is taken: sore mouth, diarrhea, stomach upset, trouble swallowing, swollen tongue, vaginal itching, mild nausea, sores or swelling in your rectal or genital area, and vomiting.