Of course, the spirit of sport has evolved considerably over the years. As cynics note, doping — the word is thought to come from “dop,” a stimulant drink consumed by the native peoples of South Africa — is as old as sport itself. Ancient Greek athletes ate hallucinogenic mushrooms; the Tarahumara, the famous running tribe of northern Mexico, goosed their endurance with peyote.

Things got even wilder with the rise of professional sport in the late 19th century, as Charles E. Yesalis and Michael S. Bahrke, the authors of “Performance Enhancing Substances in Sport and Exercise,” recount. Cyclists in grueling six-day races downed sugar cubes dipped in anesthetic ether, and sprinters looked for an explosive edge from nitroglycerin, which dilates blood vessels. Thomas Hicks’s victory in the 1904 Olympic marathon “demonstrated that drugs are of much benefit to athletes,” his doctor proclaimed, after dosing him en route with a mixture of brandy and strychnine, a toxin that is potentially fatal but was used as a stimulant in small amounts.

It was only in the 1920s that drugs began to be viewed as contrary to the spirit of sport. Stimulants were first banned by track and field authorities in 1928. In the decades since, the anti-doping authorities’ lists of banned substances and testing protocols have been repeatedly updated in an effort to catch up to new waves of drugs, like steroids in the ’60s and ’70s, and hormones such as EPO since the late ’80s.

Still, no matter how long the list of bans gets (the 2015 edition fills 10 densely packed pages), athletes have plenty of options. Steroids may be banned, but creatine is a legal muscle builder, increasing the energy stores available for short-term all-out efforts like weight lifting. Pseudoephedrine, the decongestant in over-the-counter meds like Sudafed, is a stimulant that was taken off the banned list after 2003, only to be restricted again in 2010 after the World Anti-Doping Agency’s monitoring of test results showed widespread and growing abuse.

Then there’s caffeine, the most versatile ergogenic aid of all, which fights mental fatigue and may also have effects on muscle contraction and metabolism. Even Tylenol has been shown to boost endurance performance by 2 percent.

The hottest endurance booster of the last Olympic cycle, meanwhile, was beet juice concentrate. It turns out that the nitrates in beets — the same nitrates everyone worries about in hot dogs — are converted with the help of friendly bacteria in your saliva into nitric oxide, which enables your muscles to use less oxygen when they contract. The result: a speedup of 2 percent or more in endurance races.

And these are just the ones that actually work. You don’t have to look far to find stories of athletes experimenting with everything from Viagra to hornet larvae extract in search of an edge. So where, then, is our bright moral line?