Know what your doctor is doing (part three)
Antibiotic Therapy
- Penicillin
Because of their low cost,safety,and efficiency,penicillins should be used whenever possible. Beta-lactam antibiotics inhibit cell wall peptidoglycan synthesis and are bactericidal. Resistance arises due to beta-lactamases which cleave the beta-lactam ring.
A.Penicillin (PCN). aueous,procaine and benzathine PCN G, and PCN VK.
Spectrum
PCN G is the drug of choice for S.pneumoniae,Group A and viridans streptococci ,S.bovis,penicillin-susceptible S.Aureus(rare), N.meningitidis etc.
Kinetics and dosage
PCN has excellent tissue penetration including across inflamed meninges and is cleared via the kidneys. Modify dose for renal failure.
a. High dose therapy- 18-24 million units a day given as 2 million units IV or 3-4 million units are used in serious ifections such as meningitis.
b. Intermediate dose therapy- 6-12 million units a day given as 1-2 million units IV .Used for pneumonia,synergism with aminoglycosides,and soft tissue infections.
c. Low dose therapy- Procaine PCN 600,000 units IM or aqueous PCN G 1 million units IV is effective in most cases of pneumococcal pneumonia. Treatment of syphilis,gonorrhea or streptococcal pharyngitis plus prophylaxis of rheumatic fever with benzathine PCN G are other examples.
Adverse effects and drug interactions
PCN allergy drug fever, eosinophilia and CNS side effects of myoclonus followed by seizures may occur. If allowed to mix with an aminoglycoside in IV tubing or bag,significant inactivation may occur. Probenecid decreases renal tubular secretion and may increase the half-life of PCN.
- AMPICILLIN AND AMOXICILLIN
Spectrum
Similar to PCN G.Alsoactive against H.influenzae,E.coli,Proteus sp., Salmonella sp.,and Shigella sp.
Kinetics and dosage
Kinetics are similar to PCN G with excellent levels in joints,serosal cavities,the unobstructed biliary tree,urine and CSF.High dose therapy is with 2 gm IV and low dose is 250-500 mg ampicillin PO qid. Amoxicillin,which is an oral preparation is 95% absorbed versus 40% for ampicillin,so amoxicillin is dosed 250-500 mg.Food interferes with the absorption of ampicillin but not amoxicillin.
Adverse effects
In addition to the side-effects of PCN G, morbilliform rash,nausea,vomiting and diarrhea are seen.
to be continued...
P.S. Photo is take from www.health.harvard.edu