Doxycycline × Iron (oral salts: sulfate, fumarate, gluconate, bisglycinate, polymaltose)

Iron forms non-absorbable chelates with tetracyclines in the gut (as with the doxycycline-calcium pair). Doxycycline bioavailability falls by 50–80%.

Persistent infection symptoms 48–72 hours after antibiotic start, risk of clinical failure.

Take doxycycline 2 hours before or 6 hours after iron supplements. Same interval applies to calcium, magnesium, zinc, and aluminium-containing antacids. If timing separation is impossible (e.g. severe iron deficiency), choose an alternative antibiotic (amoxicillin, a cephalosporin).