What is the safety margin on duration of an antibiotics (levofloxacin) treatment?

Question

Recently I was prescribed an antibiotics (levofloxacin) treatment of 500mg daily for 7 days. Being in Indonesia the doctors generally don't give any information and I took my first dose at exactly the same time as a high dosage vitamin/mineral supplement. Later I read that this actually may have pretty much voided the first dose entirely, see WebMD concerning "taking other products that may bind to it" or Wikipedia:

When levofloxacin is taken with anti-acids containing magnesium hydroxide or aluminum hydroxide, the two combine to form insoluble salts that are difficult to absorb from the intestines. Peak serum concentrations of levofloxacin may be reduced by 90% or more, which can prevent the levofloxacin from working. Similar results have been reported when levofloxacin is taken with iron supplements and multi-vitamins containing zinc

Is this enough reason to buy an extra dose on my own and continue for 1 more day or is there enough safety margin built into these treatments that account for some incorrect usage?

Answer 1

Unfortunately, what you read about chemical interactions of levofloxacin with polyvalent cations is correct - the medicine binds to the cation to form a coordination complex in the gastrointestinal tract, which then cannot be absorbed.

From the SPC of levofloxacine:

Levofloxacin absorption is significantly reduced when iron salts, or magnesium- or aluminium-containing antacids, or didanosine (only didanosine formulations with aluminium or magnesium containing buffering agents) are administered concomitantly with levofloxacin tablets.. Concurrent administration of fluoroquinolones with multi-vitamins containing zinc appears to reduce their oral absorption. It is recommended that preparations containing divalent or trivalent cautions such as iron salts, zinc-salts or magnesium- or aluminium-containing antacids, or didanosine (only didanosine formulations with aluminium or magnesium containing buffering agents should not be taken 2 hours before or after Levofloxacin [...] administration. Calcium salts have a minimal effect on the oral absorption of levofloxacin.*

As for the length of therapy, the same source recommends different duration, depending on the diagnosis (from 3 days for urinary tract infections up to 8 weeks for antrax; the most common therapy duration for respiratory infections is 7 to 10 days (depending on the severity).

This is the most important factor that determines for how long you should take the medicine, but there are other factors: the severity of the condition, your progress, the strain of bacteria (some are more sensitive then others) and also the dose (levofloxacine can be given by mouth in doses of 250 mg daily, but this depends on the condition, your body mass, liver and kidney function etc.).

In conclusion, I'm sorry to say, but this would be a question for your doctor.

---

*Other fluoroquinolones may bind with calcium as well.