Newborns
There has been no established safety and effectiveness for use in premature infants and neonates. Elderly
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. Additional considerations
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
Side effects associated with use of cefazolin therapy include:
Common (1-10%): diarrhea, stomach pain or upset stomach, vomiting, and rash. Uncommon (<1%): dizziness, headache, fatigue, itching, transient hepatitis.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. Those with an allergy to penicillin may develop a cross sensitivity to cefazolin. Mechanism of action
Cefazolin inhibits cell wall biosynthesis by binding penicillin-binding proteins which stops peptidoglycan synthesis.
In many instances of staphylococcal infections, such as bacteremia, cefazolin is an alternative to penicillin in patients who are allergic to penicillin. However, there is still potential for a reaction to occur with cefazolin and other cephalosporins in patients allergic to penicillin. Resistance to cefazolin is seen in several species of bacteria, such as Mycoplasma and Chlamydia, in which case different generations of cephalosporins may be more effective. Cefazolin does not fight against Enterococcus, anaerobic bacteria or atypical bacteria among others. Bacterial susceptibility
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. Spectrum of activity
Gram-positive aerobes:
Staphylococcus aureus (including beta-lactamase producing strains)
Staphylococcus epidermidis
Streptococcus pyogenes, Streptococcus agalactiae, Streptococcus pneumoniae and  other strains of streptococciGram-Negative Aerobes:

Escherichia coli
Proteus mirabilis

Non susceptible
The following are not susceptible:
Methicillin-resistant staphylococcus aureus
Enterococcus
most strains of indole positive Proteus (Proteus vulgaris)
Enterobacter spp. Morganella morganii
Providencia rettgeri
Serratia spp. Pseudomonas spp. Listeria

Special populations
Pregnancy
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.