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 of orally taken tetracycline?
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.
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.