A former headmaster who amassed thousands of indecent images of children on his computer walked free from court yesterday, after a judge ruled that the drug he had been taking to treat his Parkinson's disease was responsible for his crime.

Phillip Carmichael, 58, pleaded guilty to the offences, but received an absolute discharge – escaping any form of punishment – after claiming that the pills he had been taking turned him into a paedophile.

Police discovered 8,000 images and videos stored on the former schoolteacher's computer during a raid on his home in Wantage, Oxfordshire. But only one of them had been downloaded before he was prescribed dopamine-stimulating drugs to treat his two illnesses, Parkinson's disease and Graves' disease.

Mr Carmichael, the former headmaster of a primary school, who had an "impeccable record", claimed the medicine was known to have side effects including compulsive gambling, shopping and hypersexuality, or sex addiction. The defendant took both cabergoline and ropinirole, collectively known as dopamine agonists. Cabergoline has since been withdrawn from use.

Judge Mary Jane Mowat described the case as "wholly exceptional", and said it was clear that Mr Carmichael had not acted under his own volition.

She said: "This is a very distressing case. To say that he was to blame would be a complete denial of the reality of the evidence that I see.

"He was not only an ill man at the time, but a man whose medication can be described as ultimately responsible for the committal of these offences."

Roger Harrison, foer the defence, had earlier called a medical expert, who reported that hypersexuality was indeed a common side effect. Mr Harrison said: "It was not his fault. All he had done was contract this disease and put himself in the hands of doctors. There is no culpability."

Mr Harrison told Oxford Crown Court that his client had no idea how the pictures and videos came to be on his computer, but had started behaving strangely in other ways since he started taking the drug, such as making unnecessary purchases from shopping catalogues.

Mr Carmichael, who retired on the grounds of ill-health in 1999, was a keen painter, but after doctors prescribed him the treatment, the pictures he produced became "messy and aggressive". Experts said that this process was called "punding", and confirmed that it was probably caused by his medication.

During the case, it also emerged that he had been downloading many of the images from a website called Young Video Models, and had even written to the owners complaining about the quality of the photographs he was receiving.

The majority of the images were described as level one – the least serious category of indecent images depicting children – but one of them was level five, the most serious.

Henry James, for the prosecution, said Mr Carmichael was interviewed about the images three times by police officers, but on each occasion remained silent or said "no comment".