Doxycycline HYC 100 milligram is an antibiotic that is used to treat bacterial infections, such as periodontitis, urinary tract infections, chlamydia, acne and gonorrhea, according to RxList. It is also used to treat bumps, acne-like blemishes and bumps that are caused by rosacea.
Doxycycline HYC 100 milligram can make birth control pills work less effectively. It can also initiate upset stomach, mild diarrhea, mild nausea and faint skin itching. Severe allergic reactions to this medication can include hives, blurred vision, bloody or watery diarrhea, swelling of the face or tongue, confusion, weakness, burning of the eyes and skin pain, warns RxList.