Which he was cleared of. Twice.
Eventually he was cleared because of a loophole.
At first Guardiola claimed he had been the victim of contaminated supplements given to him by his personal physician Dr. Ramon Segura. So the authorities tested the supplements Guardiola had taken and they didn't contain Nandrolone.
What they did find though was that despite claiming contamination, Dr. Segura could not even be sure of the contents of these supplements. It was discovered during Guardiola’s unsuccessful appeal that Dr. Segura’s behaviour in preparing Guardiola’s supplements was deemed “risky”. These supplements were prepared with “raw materials purchased from different suppliers according to market availability”, without suitable “certification of manufacturers”.
So you could say that in a way Guardiola had been put at risk by Dr Segura. Obviously you would expect Guardiola to be outraged by Segura's lackadaisical approach to preparing supplements. Especially given another client Frank De Boer also tasted positive for Nandrolone.
However instead of shunning Segura and his dodgy supplements he installed him as Barca club doctor and encouraged his players to take the supplements.
When his first defence failed instead of holding his hands up Guardiola enlisted the help of another Doctor who claimed Guardiola suffered from Gilbert syndrome. A genetic condition that they argued could produce elevated levels of Nandrolone. The marker for nandrolone use in drug testing is 19-noretiocholanolone, the legal limit is 2mg and one of Guardiola's samples showed 12mg so unsurprisingly his second defence was thrown out.
Two defences down Guardiola still didn't hold his hands up. He was probably going to rely on the dog ate my home work defence until an anomaly was found. n 2005, WADA had found that a phenomenon called “unstable urine” in samples could lead to positive tests for low levels of nandrolone. In very rare cases nandrolone could be found in samples not because of external administration but as a result of a chemical reaction that “may occur in a vial containing urine.”
WADA instructed all accredited labs to perform “stability tests” on urine samples with nandrolone concentration from 2 to 10ng/ml moving forward. Guardiola’s values were at the high end of this scale (12ng/ml for NE). Those samples that were deemed “unstable” would not constitute an adverse analytic finding for nandrolone.
Then-WADA Director General David Howman stood by the efficacy of previous testing for nandrolone and said the chances of urine becoming unstable were “very rare”. The chances were between 1 out of 1,000 and 1 out of 10,000 positive tests for nandrolone.
Guardiola was cleared by the Brescia Court of Appeals. This was not because his samples were deemed “unstable” but because it could have been possible that his four samples had been “unstable”. Guardiola was absolved of all blame because of “the impossibility to now perform stability tests on the samples taken” in 2001.
Stability tests must be carried out within five weeks of the collection of a sample. In 2007, no sample even remained to be re-tested.
Yet Italian anti-doping prosecutors would appeal the decision in 2009 arguing that Guardiola should not have been allowed another appeal. The change in WADA’s guidelines did not constitute “new evidence”, they argued, because anti-doping laboratories were correctly following the testing procedures set by WADA at the time. Further they argued that Guardiola’s representatives had never contested how the sample was collected or analysed in previous cases and that this did not form part of his previous defence.
Guardiola had 4 samples of urine taken so the chance that all four were contaminated is miniscule. He was cleared because it simply wasn't possible to prove the stability of six year old urine.