Background: Antibiotic abuse has potential serious effects on adult health, including the development of antibiotic resistance, which represents one of the most important challenges in the world. Aim: This study aimed to assess the effect of antibiotic abuse on adults' health. Research design: A descriptive analytical design used to conduct this study. Setting: The study was conducted at the outpatient clinic in Specialized Hospital which affiliated to Ain Shams University in Cairo governorate. Subjects`: A purposive sample consisted of 377 adult patient who were diagnosed clinically and treated by antibiotics. Tools: Two tools were used for data collection: 1^st tool: an interviewing questionnaire for adults included three parts: I. Demographic data, II. Adults' knowledge and III. Reported practices about antibiotic abuse and 2^nd tool: Adults' health status assessment sheet consist of two parts: I. Physical examination, II. Medical history. Results: The results illustrate that the age of 37.9% of the studied sample ranged between >25 – 35 years, with Mean SD 33.4 ± 5.78, 33.4% of them had history of chronic diseases, 33.3% had cardiac diseases, and 36.6% of them used an antibiotic without a doctor's prescription. Also, the study proved that 54.4% of the studied sample had unsatisfactory level of total knowledge, and 61.5% of them had unhealthy practices about antibiotic abuse. Conclusion: The study results concluded that there was a highly significant positive correlation between total knowledge and total reported practices related to antibiotic abuse. Therefore, there was a highly statistically significant relation between total knowledge and total reported practices of the studied sample and their history of using antibiotics without a doctor's prescription with (p-value = <0.001). Recommendation: Design seminars and workshops in relevant health sectors about antibiotic misuse and the importance of antibiotic resistance addressed to adults with chronic diseases and those treated with antibiotics.