Footballers who rely on caffeine pills to boost their performance must know that such pills may cause more harm than good.
According to a World Anti-Doping Agency research, caffeine can be counterproductive on the field.
The Herald Sun quotes sports doctor Peter Larkins as saying that the threshold identified could be equal to as few as four No-Doz tablets or several cups of coffee, depending on the player's size and metabolism.
AFL players, including St Kilda's Lenny Hayes, have admitted taking up to six No-Doz in a game.
Until 2005, elevated caffeine readings were a breach of the WADA code, which the AFL follows.
WADA changed its mind after discovering "caffeine is performance-decreasing above the 12 microgram/ml threshold (in urine) that was historically used in sport".
The Australian Sports Anti-Doping Authority yesterday said caffeine remained on a WADA "monitoring program" to detect patterns of misuse.