The Russian, 28, admitted Monday to taking the illegal substance
, but wrote a letter
to her fans on Facebook Friday to saying she makes "no excuses for not knowing about the ban", which came into effect on January 1.
The five-time grand slam winner, who faces a suspension of up to four years, claims the communications from tennis authorities about changes to drugs on the banned list were "too hard to find."
Sharapova said she had been taking the heart drug
since 2006 but dismissed reports she took it every day, saying she followed doctors' instructions and took it in the "low doses recommended."
Reports this week suggested the normal course of treatment for patients on meldonium, sometimes known as mildronate, is four to six weeks.
"That headline has been repeated by many reporters who fail to tell their viewers and readers what the rest of the story says," wrote Sharapova.
"The story quotes the manufacturer of my medicine as saying: 'Treatment course can be repeated twice or thrice a year. Only physicians can follow and evaluate patient's health condition and state whether the patient should use meldonium for a longer period of time.'
"That's exactly what I did. I didn't take the medicine every day."
The World Anti-Doping Agency (WADA) began monitoring the use of meldonium by athletes in 2015, but Sharapova denies being repeatedly warned about it.
"A report said that I had been warned five times about the upcoming ban on the medicine I was taking," she wrote. "That is not true and it never happened."
Sharapova said the way the International Tennis Federation, the sport's governing body, informed players of the changes were "buried in newsletters, websites, or handouts."
She wrote: "In order to be aware of this 'warning', you had to open an email with a subject line having nothing to do with anti-doping, click on a webpage, enter a password, enter a username, hunt, click, hunt, click, hunt, click, scroll and read. I guess some in the media can call that a warning. I think most people would call it too hard to find.
"No excuses, but it's wrong to say I was warned five times."
Sharapova failed a drug test on January 26 after losing to Serena Williams in the quarterfinals of the Australian Open. She was charged with an anti-doping violation on March 2, and was provisionally banned from March 12.
The ban could be reduced to two years or less if anti-doping officials find Sharapova did not intentionally take the drug to enhance performance.