John McEnroe says he has no issue with a player taking a legal drug, even if it has performance-enhancing qualities.
The three-time Wimbledon champion's comments follow Maria Sharapova's positive test for meldonium.
Sharapova took the drug, which was added to the World Anti Doping Agency's banned list on 1 January, for 10 years.
"If a drug is legal? That is like a no-brainer. I mean, are you kidding?" McEnroe told BBC Sport on a visit to the BNP Paribas Open in Indian Wells.
"People have been looking since the beginning of time for an edge, and you're constantly looking for these things in any way, shape or form.