: Effect of milk on effectiveness of oral tetracycline Tetracycline is known to be inhibited by calcium ions, and milk contains quite some calcium ions. What I want to ask is, to what extent does drinking milk affect the effectiveness

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Calcium-rich food interferes with absorption of tetracycline considerably:

Milk and other dairy products, antacids containing polyvalent cations, as well as various iron salts ingested simultaneously with tetracycline derivatives, might interfere with their absorption by 50 to 90% or even more.

Interactions with the Absorption of Tetracyclines

However, you can't calculate with that, by for example just taking twice the dosage. The safe method is to not ingest milk or any other calcium-rich food together with the tetracycline. The recommendation appears to be to leave one to two hours between the antibiotics and milk (taking the antibiotics first) :

Do not take milk, milk formulas, or other dairy products within 1 to 2 hours of the time you take tetracyclines (except doxycycline and minocycline) by mouth

The study referenced above recommends an even more cautious three hours.