A chemistry student who killed himself by overdosing on caffeine may have lain dead in his flat for up to six days before being discovered, an inquest heard today.

James Christopher Bird, 20, was last seen alive by a friend, David Semp, at Cardiff University's Talybont halls of residence on January 24 this year.

Sergeant Warren Poole told the inquest, at Cardiff Coroner's Court, that a receipt found in the first-year student's possession showed he had bought tablets locally.

His friends thought he had gone home to visit his family in Hereford for the weekend but raised the alarm after becoming concerned when he did not arrive back and they could hear his mobile phone ringing in the room.

Mr Semp contacted security staff at the halls on January 31 and Mr Bird's body was found on the bed in a decomposed state.

Sergeant Poole told the inquest that several handwritten notes were found in the room.

One of the notes showed calculations working out how much caffeine would be needed to kill someone.

A search of Mr Bird's computer also found he had been looking at internet websites about caffeine.

One note stated: "It is not having too much effect." Another said: "I want to die."

But Sergeant Poole said that, despite a comprehensive investigation, no reasons were found for Mr Bird to take his own life.

He had been doing well in his course and was scoring especially highly in the forensic chemistry module.

Pathologist Dr Alan Gibbs told the inquest the cause of death was overdose of caffeine.

A total of 150mgs of caffeine was found in 100mls of blood, enough in Dr Gibbs' view, to cause death.

Recording a verdict of suicide, Cardiff and Vale coroner Dr Lawrence Addicott said that while Mr Bird wished to die, there did not seem to be a reason why he wanted to.

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• This article was amended on 27 January 2012. The original gave information inconsistent with more recent Guardian editorial guidelines about the coverage of suicides.