Doxycycline belongs to the group of drugs called tetracycline antibiotics. The medicines from this group are efficient in the treatment of bacterial infections, such as pneumonia, Lyme disease and a number of other respiratory tract and urinary tract infections. While some mild symptoms that you will get at the beginning of the treatment are most likely to go away as your body adjusts to the dosage and schedule of taking doxycycline, in some rare cases people report serious side effects that require you health care provider's immediate attention. Call your doctor if you notice any of the following serious side effects that should not be left unattended: severe headache, loss of appetite, confusion, extreme weakness, upset stomach, vomiting, skin rash, itching, hives, yellowing of the skin or eyes, dark-colored urine, blurred vision, and stomach pain. All the meds you are taking at the moment have to be discussed with your health care provider in advance. You are not supposed to start any new medications before consulting a qualified health professional, nor can you start your treatment with doxycycline if you are taking any of the following medications without previously taking to your doctor: antacids, vitamins, penicillin, anticoagulants, carbamazepine, phenobarbital, and phenytoin. If you have diabetes doxycycline may affect the accuracy of some tests you have done. This is why make sure you let your doctor know that you are taking doxycycline before having any tests done.