doxycycline

introduction

a tetracycline class bacteriostatic antibiotic
active against a wide range of Gram positive and Gram negative organisms, Chlamydia, Mycoplasma, Rickettsiae, Borrelia, syphilis, N. gonorrhoeae, pre-erythrocytic and asexual bloodstages of Plasmodium falciparum
tetracyclines are only partially active against the pre-erythrocytic stages of P. vivax
has no activity against the relapsing forms (hypnozoites) of Plasmodium vivax
not the drug of choice in the treatment of any type of staphylococcal infection or infections caused by Streptococcus pneumoniae, Haemophilus influenzae, Strep. pyogenes, Strep. faecalis, or any type of enteric bacteria, because many strains of these organisms have been shown to be resistant to doxycycline

contraindicated in pregnancy (safe in 1st 18 weeks though), lactation and children under 8 years age due to the risk of permanent tooth discolouration and use of tetracyclines in infants may cause increased intracranial pressure and bulging of the fontanelles, and decrease in fibula growth has been reported when given to lactating mother sof premature infants
some patients may be photosensitive and should avoid excess sunlight
patients who are on anticoagulant therapy may require downward adjustment of their anticoagulant dosage
may render combined oral contraceptive pill (OCP) less effective and cause breakthrough bleeding and unplanned pregnancies - other contraception should be used during Rx and for 7 days after Rx. Some patients do show large decreases in the plasma concentrations of ethinyl estradiol when they take certain other antibiotics, notably tetracycline and penicillin derivatives thus caution is advised ¹⁾ despite widespread opinion that there is no evidence that antibiotics other than rifampicin alter the efficacy of OCPs (assuming there is no diarrhoeal illness caused by the antibiotics which may reduce absorption of the OCP).²⁾
rare cases of benign intracranial hypertension have been reported after tetracyclines
oesophagitis and oesophageal ulceration have been reported in patients receiving doxycycline - ensure adequate amount of fluids taken during doses, and take doses in the morning rather than late evening
concurrent use of tetracycline and methoxyflurane has been reported to result in fatal renal toxicity
may exacerbate systemic lupus erythematosus (SLE)
abnormal hepatic function and cholestatic hepatitis has been reported rarely
may cause tinnitus

pharmacology

virtually completely absorbed after oral administration, not significantly affected by the presence of food or milk, but antacids or calcium, magnesium containing medications should be avoided
>90% of an oral dose of doxycycline is eliminated from the body within 72 hours of drug administration - ~40% by kidneys if normal renal function, and remainder in faeces
antianabolic action of tetracyclines may cause an increase in serum urea

dosage

usual dose for adults and children > 50kg:
- most infections: 100mg bd on 1st day then 100mg once daily or 50mg bd
- for more severe infections, 100mg bd can be used
- for acute uncomplicated gonococcal infections, 100 mg twice daily for five to seven days
- for severe acne, 50mg once daily for up to 12 weeks has been used
- malaria chemoprophylaxis:
  - 100 mg once daily, commencing two days prior to entering malarious areas, while in the malarious area and for two weeks after leaving the malarious area
children aged > 8 yrs but < 50kg: 2 mg/kg equates to the adult dose of 100mg

¹⁾

Obstet Gynecol. 2001;98(5 Pt 1):853.

²⁾

J Am Acad Dermatol. 2002;46(6):917.

Table of Contents

doxycycline

introduction

pharmacology

dosage