Side effects of doxycycline tablets include gastrointestinal discomfort, allergic reactions, hematologic abnormalities, central nervous system abnormalities, and secondary infections. 1. Gastrointestinal discomfort: such as nausea, vomiting, abdominal pain, diarrhea, esophagitis, esophageal ulcer. 2. Allergic reactions: such as maculopapular rash, erythema, urticaria, angioneurotic edema, pericarditis, anaphylactic purpura, asthma, anaphylactic shock, photosensitization phenomenon. 3. Blood system abnormalities: such as hemolytic anemia, thrombocytopenia, eosinophilia, neutropenia and so on. 4. Abnormalities of central nervous system: such as headache, vomiting, etc. 5. Secondary infections: long-term use of the drug can cause respiratory, gastrointestinal and urinary tract infections caused by drug-resistant staphylococcus aureus and other pathogens, and in severe cases, sepsis can also occur. 6. Others: such as dry mouth, xerostomia, pharyngitis, lingual inflammation, liver damage and so on. Doxycycline tablets is a kind of tetracycline antibiotics, clinically mainly used for the treatment of various sensitive pathogenic bacteria caused by diseases, such as psittacosis, regression fever, cholera, bubonic plague, etc., but also can be used for moderate and severe acne auxiliary treatment, as well as for penicillin allergy to tetanus, gas gangrene and other diseases of the treatment. It should be noted that doxycycline tablets are forbidden to be used by people with a history of allergy to tetracyclines and children under the age of 8. Pregnant women are also not recommended to apply this drug, and breastfeeding women should stop breastfeeding during the use of the drug. Patients should take doxycycline tablets according to the doctor’s instructions, during the period of taking the drug, if any of the above discomfort occurs, persists without relief or the symptoms are more serious, should consult a doctor in time.