Abstract: Objective To investigate the characteristics of and contributing factors to doxycycline-induced black hairy tongue (BHT) and to provide references for safe clinical medication. Methods One case of BHT caused by doxycycline injection was analyzed based on literature review before recommendations about medication were given. Results A child with cystic fibrosis developed pigmentation on the mid-dorsal tongue after 18 days of doxycycline infusion. The symptoms were gradually improved after discontinuation of the drug. Causality analysis, combined with a review of related literature and case reports, indicated that this adverse reaction was likely associated with doxycycline. Conclusion Doxycycline, a widely used antimicrobial agent, carries the risk of inducing BHT. Clinicians should remain vigilant, particularly in patients with oral microbiome imbalances, underlying diseases, compromised immunity, or prolonged/high-dose antibiotic use. Enhanced oral care monitoring is recommended during treatment, and prompt assessment and regimen adjustment should be offered in case of pigmentation.

Key words: Doxycycline, Children, Cystic Fibrosis, Black Hairy Tongue, Adverse Drug Reactions