Abstract: We included 183 studies with a total of 252,886 participants (range 40 to 190,238). The indications for macrolide antibiotics varied greatly, with most studies using macrolides for the treatment or prevention of either acute respiratory tract infections, cardiovascular diseases, chronic respiratory diseases, gastrointestinal conditions, or urogynaecological problems. Most trials were conducted in secondary care settings. Azithromycin and erythromycin were more commonly studied than clarithromycin and roxithromycin. Most studies (89%) reported some adverse events or at least stated that no adverse events were observed. Gastrointestinal adverse events were the most commonly reported type of adverse event. Compared to placebo, macrolides caused more diarrhoea (odds ratio (OR) 1.70, 95% confidence interval (CI) 1.34 to 2.16; low-quality evidence); more abdominal pain (OR 1.66, 95% CI 1.22 to 2.26; low-quality evidence); and more nausea (OR 1.61, 95% CI 1.37 to 1.90; moderate-quality evidence). Vomiting (OR 1.27, 95% CI 1.04 to 1.56; moderate-quality evidence) and gastrointestinal disorders not otherwise specified (NOS) (OR 2.16, 95% CI 1.56 to 3.00; moderate-quality evidence) were also reported more often in participants taking macrolides compared to placebo. The number of additional people (absolute difference in risk) who experienced adverse events from macrolides was: gastrointestinal disorders NOS 85/1000; diarrhoea 72/1000; abdominal pain 62/1000; nausea 47/1000; and vomiting 23/1000. The number needed to treat for an additional harmful outcome (NNTH) ranged from 12 (95% CI 8 to 23) for gastrointestinal disorders NOS to 17 (9 to 47) for abdominal pain; 19 (12 to 33) for diarrhoea; 19 (13 to 30) for nausea; and 45 (22 to 295) for vomiting. There was no clear consistent difference in gastrointestinal adverse events between different types of macrolides or route of administration. Taste disturbances were reported more often by participants taking macrolide antibiotics, although there were wide confidence intervals and moderate heterogeneity (OR 4.95, 95% CI 1.64 to 14.93; I² = 46%; low-quality evidence). Compared with participants taking placebo, those taking macrolides experienced hearing loss more often, however only four studies reported this outcome (OR 1.30, 95% CI 1.00 to 1.70; I² = 0%; low-quality evidence). We did not find any evidence that macrolides caused more cardiac disorders (OR 0.87, 95% CI 0.54 to 1.40; very low-quality evidence); hepatobiliary disorders (OR 1.04, 95% CI 0.27 to 4.09; very low-quality evidence); or changes in liver enzymes (OR 1.56, 95% CI 0.73 to 3.37; very low-quality evidence) compared to placebo. We did not find any evidence that appetite loss, dizziness, headache, respiratory symptoms, blood infections, skin and soft tissue infections, itching, or rashes were reported more often by participants treated with macrolides compared to placebo. Macrolides caused less cough (OR 0.57, 95% CI 0.40 to 0.80; moderate-quality evidence) and fewer respiratory tract infections (OR 0.70, 95% CI 0.62 to 0.80; moderate-quality evidence) compared to placebo, probably because these are not adverse events, but rather characteristics of the indications for the antibiotics. Less fever (OR 0.73, 95% 0.54 to 1.00; moderate-quality evidence) was also reported by participants taking macrolides compared to placebo, although these findings were non-significant. There was no increase in mortality in participants taking macrolides compared with placebo (OR 0.96, 95% 0.87 to 1.06; I² = 11%; low-quality evidence). Only 24 studies (13%) provided useful data on macrolide-resistant bacteria. Macrolide-resistant bacteria were more commonly identified among participants immediately after exposure to the antibiotic. However, differences in resistance thereafter were inconsistent. Pharmaceutical companies supplied the trial medication or funding, or both, for 91 trials. The macrolides as a group clearly increased rates of gastrointestinal adverse events. Most trials made at least some statement about adverse events, such as "none were observed". However, few trials clearly listed adverse events as outcomes, reported on the methods used for eliciting adverse events, or even detailed the numbers of people who experienced adverse events in both the intervention and placebo group. This was especially true for the adverse event of bacterial resistance.

Plain-language summary: We included 183 studies with a total of 252,886 participants. Most studies were conducted in the hospital setting. Azithromycin and erythromycin were more commonly studied than clarithromycin and roxithromycin. Most studies (89%) reported some adverse events, or at least stated that no adverse events were observed. Drug companies supplied trial medications or funding, or both, in 91 studies. Funding sources were unclear in 59 studies. People treated with a macrolide antibiotic experienced gastrointestinal adverse events such as nausea, vomiting, abdominal pain, and diarrhoea more often than those treated with placebo. Taste disturbances were reported more often by people taking macrolides than those taking a placebo. However, as very few studies reported on these adverse events, these results should be interpreted with caution. Hearing loss was reported more often by people taking macrolide antibiotics, however only four studies reported this outcome. Macrolides caused less cough and fewer respiratory tract infections than placebo. We did not find any evidence that macrolides caused more cardiac disorders, liver disorders, blood infections, skin and soft tissue infections, changes in liver enzymes, appetite loss, dizziness, headache, respiratory symptoms, itching, or rashes than placebo. We did not find more deaths in people treated with macrolides than in those treated with placebo. Very limited information was available to assess if people treated with a macrolide antibiotic were at greater risk of developing resistant bacteria than those treated with placebo. However, bacteria that did not respond to macrolide antibiotics were more commonly identified immediately after treatment in people taking a macrolide than in those taking a placebo, but differences in resistance thereafter were inconsistent. The quality of the evidence ranged from very low (cardiac disorders, change in liver enzymes, liver disorders) to low (abdominal pain, death, diarrhoea, dizziness, hearing loss, skin and soft tissue infections, taste disturbance, wheeze) to moderate (appetite loss, blood infection, cough, fever, headache, itching, nausea, rash, respiratory symptoms, respiratory tract infections, vomiting).