Russian tennis star Maria Sharapova has admitted to taking the banned substance Meldonium after ignoring changes to anti-doping rules.

But who declares which drugs are banned and how do athletes learn of the changes?

Here's a guide to how drug-use is monitored in sport:

Who declares which drugs are banned?

The World Anti-Doping Agency (Wada) issues an annual banned list of substances applicable across all sports.

The international independent agency - funded by sports and world governments - published its latest list on September 29 last year.

Meldonium was added to the list of "substances and methods ... prohibited at all times ... both in and out of competition" because of "evidence of its use by athletes with the intention of enhancing performance”.

How does Wada communicate with athletes?

Sharapova said Wada sent her an email on December 22 to inform her of the changes to the 2016 banned list, which were to take effect from January 1 2016.

She said she failed to "click" on a link to the email attachment which would have shown that the drug she had used for a decade was now off-limits.

It had already been added to the list of drugs being monitored in 2015 prior to being outlawed.

How are athletes tested?

In and out of competition by registered testers attached to each sport. Sharapova provided an anti-doping sample to testers from the Tennis Anti-Doping Programme (TADP) on January 26 at the Australian Open shortly after her quarter-final defeat to Serena Williams.

The sample was analysed by a Wada-accredited laboratory, which found traces of the now-banned meldonium. Sharapova was charged on March 2 with an Anti-Doping Rule Violation.

What action can be taken?

The International Tennis Federation (ITF) will issue a final decision on recommended punishment for Sharapova, which will be reviewed by Wada.

Wada said it will "subsequently decide whether or not to use its independent right of appeal to the Court of Arbitration for Sport (CAS)".