A Russian state medical agency says it has found new and improved alternatives to meldonium, the banned substance for which tennis star Maria Sharapova tested positive.

Federal Medical-Biological Agency head Vladimir Uiba says Russia has found "several drugs which are not banned and work significantly better than meldonium," in comments carried by Russian news agencies.

Uiba didn't name the new drugs and it wasn't immediately clear whether they are already being used by top Russian athletes. Uiba's agency is tasked with providing medical support to Russian national teams in many sports.

Sharapova is due to make her return to the court next week (Getty)

Sharapova was among over 100 athletes who tested positive after the heart drug meldonium was banned in sport last year.

Most of those were cleared because of evidence they had stopped taking meldonium before it was banned, though Sharapova was suspended because she had taken it after the cutoff date.

Numerous claims have been made over recent decades about meldonium, which is marketed for sufferers from heart and circulatory conditions, including that it can increase physical and mental endurance.

Packets of Mildronate (Meldonium) medication capsules (AFP/Getty)

However, Russian officials have said it is not performance-enhancing in a sports context, and argued it prevents heart attacks under extreme stress.

Sharapova said last year she used meldonium for 10 years for reasons including a magnesium deficiency, irregular heart test results and a family history of diabetes.