"Zero Tolerance for Doping" is the official slogan of the Olympic Games in Beijing. But that hasn't stopped anti-doping experts from expressing serious doubts about an effective crackdown on drug cheats, particularly in disciplines such as track and field, cycling, swimming and weightlifting.

Ulrike Spitz, spokeswoman for Germany's national anti-doping agency NADA, pointed out that the last Olympic Games in Athens were also hit by doping scandals.

"I think these Olympic Games will be pretty normal," Spitz said, adding however she was confident that the drug tests in Beijing would work correctly. "But it's simply really difficult to say what's going on in this huge Chinese market," Spitz said.

Gene doping and copycat drugs

Persistent rumors about a huge doping racket in China fuelled by the country's ambitions of greater sporting glory were partly confirmed last week by an explosive new documentary broadcast on German television station ARD.

The film, shot by a German reporter with a hidden camera, shows a Chinese doctor offering stem-cell therapy to athletes. Although Chinese officials have offered evidence in recent months that they are cracking down on doping before the Beijing Olympics, the TV documentary suggests China is still a major center for the production and distribution of performance-enhancing drugs.

Earlier in July, the BBC reported had seen evidence that laboratories in China were classing positive tests for the blood-boosting drug EPO as negatives. The investigation also found that copycat versions of the EPO drug are available on the Internet for as little as $50 and according to experts, often undetectable.

And in June this year, two of China's athletes, a swimmer and a freestyle wrestler, were banned for life for doping offences dealing an embarrassing blow to the host nation in the run-up to the games.

Clean competitions a farce?

Despite mounting evidence of illegal drug use in sports, Chinese officials point out that they have done much to clamp down on doping.

"Chinese sports authorities have done a lot within the framework of the anti-doping campaign," said Sun Weide, press spokesman for the Chinese Olympic Organizing Committee, who pointed out that Beijing passed an anti-doping law in 2004. "I think we have the means to deal with this problem."

That problem is by no means confined to China. This week alone, five female members of Russia's Olympic team were suspended by the international governing body for track and field and a Jamaican sprinter was dropped from his team after testing positive for a muscle-building veterinary drug.

Last month, Bulgaria withdrew its entire weightlifting team after 11 athletes tested positive for steroids, and 11 Greek weightlifters were suspended for doping.

Professional sport without doping is hardly imaginable any more.

"I never speak of clean competitions," NADA's Spitz said. "There's never a guarantee that sports events are clean. You'll always have athletes who will try to cheat. It's extremely important that detection techniques become better to ensure that the rules are upheld."

More and better tests the answer?

The International Olympic Committee has already said it plans to collect 4,500 blood and urine samples before and during two weeks of athletic events at this year's Olympic Games -- a 25 percent increase from the 2004 Athens Games.

NADA, which is responsible for testing the German squad of 450 athletes at the Olympics, is also increasing the number of tests this year to 9000 instead of the 4,800 checks it carried out in 2007.

"Every German athlete traveling to Beijing will be tested at least once," Spitz said, adding the tests varied according to the sport. "There are sport disciplines that are more risky than others, we naturally have to take that into account," she said.

The hope is that frequent testing and more sophisticated detection techniques will keep the games comparatively free of drug-cheating.

"There's simply much more money being pumped into drug research than before. For instance, now the test for the growth hormone is ready," said Spitz, adding that it would be used in Beijing. "That way you further limit the number of substances," she said. "But we're not naïve, we know there are drugs that can't be detected."