IG, a 25-year-old female patient, approaches the pharmacy counter with an arm full of OTC products. She tells her pharmacist that she is there to pick up the antibiotic cephalexin for an upper respiratory infection.

As the pharmacist rings her up on the cash register, IG asks one of the most common questions that patients pose to pharmacists: “Can I take all of my pills at the same time?”

As the pharmacist place her items in a bag—vitamin C, zinc capsules, cough syrup, guaifenesin with dextromethorphan, ibuprofen, vitamin D—the pharmacist suddenly realizes that there is a very important drug interaction with her medication present among these OTC products.

The pharmacist then responds by thanking the patient for asking such an excellent question, as one of her natural products could potentially inactivate her antibiotic, rendering it useless.

Mystery: Which natural product frequently inactivates antibiotics?

Solution: Zinc.

Divalent and trivalent cations, such as zinc, calcium, iron, magnesium, and aluminum, have a positive charge on them when they dissolve in stomach fluid. This positive charge is attracted to other molecules that have a negative charge, which then cause them to act like 2 magnets colliding together in the stomach fluid. When this happens, both the zinc and the antibiotic molecules become inactive.

The most noticeable antibiotics effected are tetracycline and quinolone antibiotics. However, the potential for drug interactions between medicines and OTC products are endless.

There is emerging evidence for the occurrence of interactions between OTC products and cephalexin, doxycycline, and antivirals. For this reason, it is beneficial to tell patients to take minerals at least 2 hours before or 4 to 6 hours after any sort of medication.

Many people take zinc to strengthen their immune system when they are sick, but if they are also on antibiotics or antivirals, the zinc could render these medications inactive, making them useless for the patient.

  1. Ding Y, Jia YY, Li F, et al. The effect of staggered administration of zinc sulfate on the pharmacokinetics of oral cephalexin. Br J Clin Pharmacol. 2012;73(3):422-7. doi: 10.1111/j.1365-2125.2011.04098.x.
  2. Penttilä O, Hurme H, Neuvonen PJ. Effect of zinc sulphate on the absorption of tetracycline and doxycycline in man. Eur J Clin Pharmacol. 1975;9(2-3):131-4. doi: 10.1007/BF00614009.    
  3. Rock AE, DeMarais PL, Vergara-Rodriguez PT, Max BE. HIV-1 Virologic Rebound Due to Coadministration of Divalent Cations and Bictegravir. Infect Dis Ther. 2020;9(3):691-696. doi: 10.1007/s40121-020-00307-4.