Dr. Gary Wadler, a prominent authority on the use of performance-enhancing substances and their effects on athletes, died on Tuesday in Port Washington, N.Y. He was 78. Nancy Riseman Wadler, his wife, said the cause was multiple system atrophy, a degenerative neurological disorder. Dr. Wadler was among the doctors, scientists, and regulators who emerged in the 1980s and ’90s as sports organizations struggled to keep pace with illicit drug use by athletes. His articulate explanations about steroids and human-growth hormones made him a frequent voice in the news media, at medical conferences, before congressional panels and at trials as an expert witness. He was equally concerned with medical issues and ethics and how the two intertwined in sports. In 1999, he told the Senate Commerce Committee that performance-enhancing drugs could undermine the Olympic movement and that young athletes could not grow to adulthood believing in “the power of chemical manipulation rather than the power of character.” To eliminate sports doping, he added, “there can be no compromise, no middle ground, no rhetorical acrobatics.”