Cefazolin does not fight against Enterococcus, anaerobic bacteria or atypical bacteria among others. As a first-generation cephalosporin antibiotic, cefazolin and other first-generation antibiotics are very active against gram-positive bacteria and some gram-negative bacteria. Their broad spectrum of activity can be attributed to their improved stability to many bacterial beta-lactamases compared to penicillins. Gram-positive aerobes:  Gram-Negative Aerobes:  The following are not susceptible:   Cefazolin is pregnancy category B, indicating general safety for use in pregnancy. Caution should be used in breastfeeding as a small amount of cefazolin enters the breast milk. Cefazolin can be used prophylactically against perinatal Group B streptococcal infection (GBS). Although penicillin and ampicillin are the standard of care for GBS prophylaxis, penicillin-allergic women with no history of anaphylaxis can be given cefazolin instead. These patients should be closely monitored as there is a small chance of an allergic reaction due to the similar structure of the antibiotics. There has been no established safety and effectiveness for use in premature infants and neonates. No overall differences in safety or effectiveness were observed in clinical trials comparing elderly and younger subjects, however the trials could not eliminate the possibility that some older individuals may have a higher level of sensitivity. People with kidney disease and those on hemodialysis may need the dose adjusted. Cefazolin levels are not significantly affected by liver disease. As with other antibiotics, cefazolin may interact with other medications being taken. Some important drugs that may interact with cefazolin such as probenecid. Side effects associated with use of cefazolin therapy include: Patients with penicillin allergies could experience a potential reaction to cefazolin and other cephalosporins. As with other antibiotics, patients experiencing watery and/or bloody stools occurring up to three months following therapy should contact their prescriber. Like those of several other cephalosporins, the chemical structure of cefazolin contains an N-methylthiodiazole (NMTD or 1-MTD) side-chain. As the antibiotic is broken down in the body, it releases free NMTD, which can cause hypoprothrombinemia (likely due to inhibition of the enzyme vitamin K epoxide reductase) and a reaction with ethanol similar to that produced by disulfiram (Antabuse), due to inhibition of aldehyde dehydrogenase.