Average Serum Half Life Of Antimicrobial Agents And Choice Of Antibiotics For Common Infections

Antibiotics or antimicrobial agents can have specific and different functions. Hence, their specificity in administration in patients is very important and vital in drug therapy. Based on Wikipedia, Serum half life of a drug, or also The biological half-life or elimination half life of a substance is the time it takes for a substance (for example a metabolite, drug,signalling molecule, radioactive nuclide, or other substance) to lose half of its pharmacologic, physiologic, or radiologic activity. Here are some antimicrobial agents and their serum half Life in different routes of administration.

In order of the routes of administration, the first is oral, the second is intramuscular and the third is intravenous, all in hours.
1. Benzyl penicillin: 4.0, 0.5, 0.15
2. Phenoxymethyl penicillin: 2.0, _, _
3. Procaine penicillin: _, 18.0, _
4. Ampicillin: 5.0, 4.0, 1.5
5. Erythromycin (appropriate preparation): 3.0, 2.5, 1.0
6. Tetracycline: 8.5, _, 8.5
7. Doxycycline: 20.0, _, 18.0
8. Chloramphenicol: 3.5, 6.0, 3.5
9. Rifampicin: 3.3, _, 3.3
10. Trimethoprim: 12.0, _, 10.0
11. Sifamethoxazole: 12.0, _, 10.0

Choice of antibiotics for common infections

Here are some drugs which are the best choices to their corresponding diseases stated here. The first drug indicates the proven value (conventional antibiotics) while the second drug indicates the possible value of cephalosporins.

Respiratory tract infections

1. Pharyngitis: Benzylpenicillin, Cephradine
2. Otitis media: Amoxycillin, Cefaclor
3. Necrotising otitis: Ticarcillin, Cefsulodin, Cefrazidime
4. Pneumonia (Streptococcus Pneumonia): Benzylpenicillin, First generation cephalosporins
5. H. influenzae: Ampicillin, (cefuroxime, cefotaxime, cephamandole)
6. Staphylococcus aureus: (Nafcillin, cloxacillin) (cephalothin, cefazolin)
7. Klebsiella: Cefazolin, (cefotaxime, moxalactam)
8. Pseudomonas: Ticarcillin, ceftazidime
9. Anaerobic organisms: (penicillin, clindamycin); (cefoxitin, moxalactam)

urinary tract infections

1. Domiciliary: Ampicillin
2. Hospital-acquired entero-bacteriaceae: Aminoglycosides, (cefotaxime, moxalactam)
3. Pseudomonas: Aminoglycosides, ceftazidime


1. Neonatal: (Ampicillin, gentamicin), (Cefotaxime, moxalactam + ampicillin)
2. Strptococcal Pneumoniae: Benzylpenicillin, Cefotaxime
3. N. Meningitidis: benzylpenicillin, cefotaxime
4. H. Influenzae: (ampicilin, chloramphenicol), (Cefotaxime, moxalactam)
5. Enterobacteriaceae: Aminoglycosides, (Cefotaxime, moxalactam)

Bone and Joint infections

1. Stapjylococcus aureus: (Nafcillin, cloxacillin), cefazolin
2. Enterobacteriaceae: Aminoglycoside, (Cefotaxime, moxalactam)
3. Pseudomonas: (Ticarcillin + aminoglycosides), (Cefsulodin, Ceftazidime)

Diarrhoeal diseases

1. Shigella, salmonella and campylobacter: Ampicillin, None

Gynaecological infections

1. Endometritis, salpingitis: (Clindamycin + gentamicin), (Cefoxitin, cefotaxime, moxalactam)

sexually transmitted diseases

1. Gonorrhoea: Penicillin, (Cefoxitin, Cefotaxime)
2. Intra abdominal infections: (Clindamycin + gentamicin), (cefoxitin, cefotaxime, moxalactam)


1. Home acquired: (Ampicillin + aminoglycosides, Ticarcillin + aminoglycosides), Moxalactam.

In most cases a combined drug therapy is for efficiency of drug to fighting the organisms involved. And also, the group of drugs in one bracket indicates drugs for a particular value. Just as stated earlier, the first value is for the proven values (conventional antibiotics) and hence, if the bracket is the first, then the drugs represents the conventional antibiotics best for that particular disease. But if the bracket is the second, then the drugs in it represent the possible value of cephalosporins.

According to Wikipedia, Cephalosporins are a class of ²-lactam antibiotics originally derived from Acremonium, which was previously known as “Cephalosporium”.
Together with cephamycins they constitute a subgroup of ²-lactam antibiotics called cephems.