Asthma drug salbutamol, for which four time Tour de France winner Chris Froome returned a test revealing twice the permissible amount, is widely used in cycling circles.

What is salbutamol?

Widely prescribed as Ventolin in inhaler form to treat asthma as a fast action method of opening airways to and from the lungs at the onset of breathing difficulties. It is banned in sports by the World Anti-doping Agency (WADA).

Why is it banned?

Salbutamol is part of a class of drugs called Beta 2 agonist which can have anabolic, or tissue building, effects. Scientific studies on whether, apart from alleviating asthma, inhaled salbutamol helps athletes are not unanimous: some say there is a tiny benefit, some say there is none.

Froome is asthmatic, what did he do wrong?

WADA accepts that asthmatics should be allowed to use inhalers. It no longer requires sufferers to produce a therapeutic use exception from a doctor. Instead WADA has set a ceiling for the amount of the drug that can show up in tests.

Froome's problem is that he exceeded the accepted level of salbutamol of 1000ng per millilitre in a urine sample on September 7 in one of around 20 such tests he gave during the Tour of Spain, which would be the equivalent of around four or five puffs on an inhaler. His reported level was 2000ng/ml.

Why did Froome test over the limit?

Froome says his asthma became aggravated during La Vuelta but insisted: "as always, I took the greatest care to ensure that I did not use more than the permissible dose."

His team Sky was quick to point out that Froome had not has not returned a "positive control". The term that those involved are using is "Adverse Analytical Finding."

What is an "Adverse Analytical Finding"?

Because there are legitimate reasons why an athlete took the substance WADA's rules require further investigations before pronouncing guilt. That means Froome is not automatically banned.

What happens next?

World cycling's governing body, the UCI, said in a statement that it notified Froome on September 20. Sky said that the finding triggered a request from the UCI "aimed at establishing what caused the elevated concentration of Salbutamol." The UCI said "proceedings are being conducted in line with the UCI anti-doping rules."

In the past cyclists have attempted to recreate the circumstances of the test to prove that the results were produced by what Sky called the "unpredictable variations in the way Salbutamol is metabolised and excreted."

Are there precedents?

The verdicts and punishments vary and the process can take a long time, but previous cases suggest that, whatever the verdict, Froome could well ride in next year's Tour.

In May 2014 at the Giro d'Italia, Diego Ulissi gave a sample that contained 1900 ng/ml – slightly lower than Froome’s. The Italian tried to replicate the results in testing in Switzerland to show it was natural. The UCI did not hold a final hearing until January 2015 when Ulissi was handed a back-dated nine-month ban.

At the Giro in 2007, two more Italians, Alessandro Petacchi and Leonardo Piepoli, returned samples containing more than 1,300 ng/ml and received different levels of punishment.

The Court of Arbitration for Sport accepted Petacchi’s argument that he had not used the substance with the intention of enhancing his performance, he was nevertheless given a retrospective 10-month ban and stripped of three stage wins in that Giro.

Piepoli, who had previous doping convictions, argued that he needed salbutamol to treat an allergy and was cleared.