Sport, at international and national levels, seems to constantly be in a doping crisis. It may be time to consider legalising performance enhancers because zero tolerance is clearly not working.
This week, the second-fastest runner of all time, Tyson Gay, reportedly tested positive for a banned substance along with the Jamaican sprinters Asafa Powell and Sherone Simpson, making for shocked headlines across the world.
And this is just one such high-profile story across numerous sports and countries. In athletics, 24 Turkish athletes are confirmed to have tested positive this year; the Australian Football League is still reeling from the ongoing Essendon scandal; and over in the United States, inquiries into an anti-ageing laboratory said to supply human growth hormone to top baseball players continues.
While the 100th Tour de France is so far untainted by positive tests, cycling doping cases have continued this year with two Giro D'Italia riders testing positive.
But there's still a sense that we are just seeing the tip of the iceberg.
British cyclist Chris Froome, who is now tested at the end of each Tour de France stage as the yellow jersey holder, has been relentlessly hounded over whether his recent impressive performances were because of doping.
We don't know who is doping and who is not. What we do know is that the zero tolerance ban on doping has failed.
The "war on doping" has seen several false victories. In 2000, the first tests for the substance EPO were introduced. (EPO is short for erythropoietin, which is a naturally occurring hormone found in the blood - athletes use the artificial peptide recombinant EPO to stimulate red blood cell production for improving oxygen transfer and boosting endurance or recovery from anaerobic exercise.)
In 2007, Pat McQuaid, head of Union Cycliste Internationale (the cycling association that oversees competitive cycling events internationally), declared biological passports "a new and historic step in the fight against doping".
Autologous blood tests were all but announced for the 2012 Olympics, but have apparently still not been implemented.
The science of drug testing has progressed, but it appears that the dopers are always a step ahead.
American Lance Armstrong is a case in point. He was tested in competition and out of competition, before and after EPO tests were implemented and before and after biological passports were introduced.
But he was only caught through the forced testimony of his teammates, who turned him in for the chance to continue their own careers as confessed dopers. And many of them are still riding at the elite, professional level.
The decision of the Spanish court to destroy evidence from the trial of Eufemiano Fuentes (a sports doctor found guilty of providing cyclists with blood doping) means we may never know who was involved with that particular clinic.
But it is thought to include clients from athletics, tennis and soccer as well as cyclists.
Leading expert on performance-enhancing drugs Werner Franke pointed out just before the last Olympics that half of the men's 100m finalists in two previous Olympics were later reported to have been doping.
Less than a year after the London Olympics, if Gay and Powell's tests are confirmed, we will be halfway to the same level at the 2012 final. A third member of the eight-man line-up, Justin Gatlin, was previously banned for doping.
Time and again, we are told the culture has changed. But the doping cases keep coming, and performances keep improving. The 2012 Olympics saw 66 Olympic records and 30 world records set.
We reached the limits of human performance in sprinting about 15 years ago; the limit for a man running 100m seems to be about 9.7 seconds. Ben Johnson ran that distance in 9.79 seconds in 1998 but was doping.
In 2005, American Tim Montgomery, a former 100m world record-holder, was banned for doping after running 9.78 seconds in 2002. In 2006, Gatlin, the defending 100m Olympic champion, was banned for doping three weeks after equalling the world record (9.77 seconds at the time).
In 2009, Jamaican Yohan Blake got a three-month ban for doping. Two years later, he became world champion in the 100m sprint and won a silver medal in the last Olympics.
He co-owns the second-fastest time in history alongside the recently banned Gay (9.69 seconds).
In 2011, Steve Mullings, of Jamaica, was hit with a lifetime ban following a positive test for a masking product after having run a personal best of 9.8 seconds.
It seems that if you are running 100m in about 9.7 seconds, you are likely to be taking performance enhancers.
To keep improving, to keep beating records, to continue to train at the peak of fitness, to recover from the injury that training inflicts, we need enhanced physiology.
Spectators want faster times and broken records, so do athletes.
But we have exhausted the human potential.
Is it wrong to aim for zero tolerance and performances that are within natural human limits? No, but it is not enforceable.
The strongest argument against doping is safety.
But anything is dangerous if taken to excess; water will kill you if you drink enough of it.
Over the past 20 years, sport has shown that performance enhancers can be administered safely. They could be administered even more safely if doping was brought into the open.
Of course there is no such thing as risk-free sport. But we need a balance between safety, enforceability and spectacle.
Consider cycling competitions - they show that elite sport is fundamentally unsafe, as Team Sky's Edvald Boasson Hagen and Geraint Thomas, both nursing fractures from recent cycling crashes, can tell you.
It was entirely appropriate to enforce the wearing of helmets to limit the safety risks.
But it would be inappropriate to limit the race to only straight, wide roads, or to remove downhill racing or to take any number of other measures that would increase safety but ruin the sport as a spectacle and cultural practice.
It would be a waste of time to take other measures, such as limiting the amount of time or the speed that riders can train at, even on the grounds of safety. It could not be enforced.
Enforceability requires a reasonable limit. If we set the maximum speed limit for cars to 20km/h, it would be safer. Many, perhaps most, of the people who died on the roads in any given year would be saved. But more people would speed.
We need to find a workable, enforceable balance.
A second, good objection lies in the nature of the intervention. If a substance came to dominate the sport and override its value, that would be a good reason to ban it.
If boxers could feel no fear, for instance, or if archers could be given rock-steady hands, it should not be permissible.
But if a substance allows safer, faster recovery from training or from injury, then it does not interfere with the sport.
We are confused, and often emotional, about doping. The word "drugs" brings to mind substances such as Ecstasy or cocaine or heroin. But most doping today uses natural substances that are involved in normal human physiology and naturally vary from time to time and person to person.
Testosterone, blood and growth hormone are all endogenous substances (that occur naturally within the body), which are banned.
Drugs such as caffeine are exogenous (not naturally occurring in the body) and effective in increasing performance, but allowed.
Taking the drug EPO increases hematocrit (ratio of red blood cells to total blood volume) levels, and is banned. Sleeping in a hypoxic air tent has the same effect but is perfectly legal.
Athletes are using these substances to optimise their own physiology, just as they do with diet, trying to maximise fluids and glucose at the right times. Confessed doping cyclist Tyler Hamilton claims to have lost a race due to failing to take a 100-calorie energy gel at the correct time (despite the fact he was also doping) in his book The Secret Race.
All of these variables are themselves affected by training at elite levels. Over the course of the Tour de France, a cyclist would lose their natural levels of red blood cells from the immense effort.
Training is about optimising human physiology, whether by changing the diet to influence the availability of glucose and glycogen, or by taking EPO in order to increase the availability of oxygen.
The risks of doping have been overstated, and zero tolerance represents the kind of unreasonable limit that is destined to be ignored by athletes. It's time to rethink the absolute ban and instead to pick limits that are safe and enforceable.
• Julian Savulescu is a visiting professor at Monash University.