A German court on Tuesday handed a life sentence to a 38-year-old former nurse for the murder of three of his patients by injecting an insulin overdose.

The nurse, who often worked in households caring for elderly patients, had access to insulin as he is a diabetic.He refused to give evidence in court but at the end of the trial apologised to relatives of his victims.

"What I did was very cruel and remains cruel," he said.

In sentencing, the man, who is a Polish citizen, the court in Munich said the severity of his crimes effectively ruled out early release after 15 years as is normally the case with life sentences in Germany.

The initial charges concerned six murders, but the prosecution reduced the number to three. In other cases, charges of attempted murder and causing grievous bodily harm were brought.

In four further cases, the prosecution called for acquittal, as there was no proof that the insulin injections had led to death. This decision led to sharp criticism from lawyers representing families of the deceased.

The public prosecutor's office essentially cited convenience as a motive, saying the accused sometimes simply did not feel like taking care of his patients at night.

He was also accused of stealing items including money, wine, detergent, toilet paper and toilet brushes.

Once, according to the prosecution, he is said to have asked following the death of one of his presumed victims whether he could have his cell phone and other valuables, "since the injured party would no longer need them now."

The man was a mechanic and trained locksmith, but took up nursing in Germany in 2015 after taking a 120-hour course, prosecutors said.

The case is one of a number of recent similar cases, the most prominent of which concerned serial murderer Niels Hoegel, who was sentenced to life in prison after being found guilty of killing 85 patients while working as a hospital nurse, also by administering lethal injections.