Doxycycline

• Chlamydia (non-pregnant — 1st-line, BASHH)
• PID outpatient regimen (non-pregnant)
• Syphilis in penicillin allergy (non-pregnant)

Binds 30S ribosomal subunit → blocks aminoacyl-tRNA → bacteriostatic. Broad spectrum incl. intracellular.

• Tooth discolouration if used >14/40
• Bone growth inhibition (calcium chelation)
• Use azithromycin in pregnancy

Chlamydia (non-preg)

100 mg PO BD × 7 d

PID (non-preg)

100 mg PO BD × 14 d

• AbsolutePregnancy (all trimesters)
• AbsoluteChildren <8 y
• CautionSevere hepatic impairment

• Photosensitivity
• Oesophagitis (take upright with water)
• GI upset
• Permanent tooth staining in offspring/children

• • AVOID throughout pregnancy — yellow-brown tooth staining + fetal bone growth inhibition (chelates Ca²⁺)
• • Avoid in lactation (limited Ca chelation in milk but theoretical risk)

• Antacids / Fe / Ca / dairy

  Caution

  Chelation → ↓ absorption

• Warfarin

  Caution

  ↑ INR

• Retinoids (isotretinoin)

  Avoid

  Idiopathic intracranial HTN

• 💡 Substitute azithromycin 1 g PO for chlamydia in pregnancy
• 💡 Inadvertent T1 exposure: no proven malformation risk — staining only with T2/T3 use