Rachel Heath, 31, of Woolston, Southampton, had continually denied attempting to murder widow Kathleen Corfield and a second offence of administering diamorphine.
Mrs Corfield, a retired civil servant, was receiving treatment for terminal cancer and died in Southampton General hospital in December 1994.
Minutes before she was due to appear in the dock at Winchester Crown Court, Mrs Heath, a home-help who had cared for Mrs Corfield, was told that the hearing was not going to proceed. She wept and hugged friends, but left refusing to make any comment.
Philip Mott QC, for the prosecution, said the decision had been taken to offer no evidence against the defendant and she should be found not guilty. He explained the Crown Prosecution Service acted in accordance with a set of rules which made it clear there were two stages in a prosecution.
The first stage was an evidential test but even if a case passed that one, prosecutors had to decide if the prosecution was in the public interest.
He said the decision not to proceed was not caused by any re-assessment of the evidence, although it was fair to say the defendant had always denied any involvement in any illegal action. There was a balance of public interest factors, including a likely sentence. If a court was likely to impose a small or nominal penalty, that may be a major factor against prosecution in a trial which could take a long time, involving witnesses with emotional involvement and being an emotional experience for the defendant herself.
That was considered before a trial, but before a trial the prosecution could only have in mind a bracket of what a judge might consider by way of sentence. Mr Mott told Mr Justice Ognall: "Your lordship has very kindly made certain views known about your assessment of the evidence, even if we prove what is set out, those views must go into the balance and cause a consideration of the public interest."
He said the decision, made at a high level in the CPS, had not taken lightly.
Mr Justice Ognall said the case involved very unusual circumstances. "It should be clearly understood neither the investigating nor prosecuting agencies are to be faulted in any way."Reuse content