Following three botched executions in which it was clear that the victims did not die immediately, questions are being raised in the US about the efficacy and legality of lethal injections.

Far from being a quick and painless means of lawfully killing a condemned prisoner, the method has been condemned as "cruel and unusual".

One of the reasons cited is a shortage of the necessary drugs due to an EU ban of 2011 on the export of medicinal products used in executions, on the grounds that "the [European] Union disapproves of capital punishment in all circumstances and works towards its universal abolition".

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Another reason has been a growing unwillingness of health professionals to participate in the legal killing of convicted prisoners even though their presence is required, albeit protected by anonymity.

Under guidelines laid down by the American Medical Association, a physician can be present at an execution and can confirm the death but "as a member of a profession dedicated to preserving life when there is hope of doing so, should not be a participant in a legally authorised execution".

As a result, there has been a sharp rise in the number of executions which have gone badly wrong. For example, on April 29 this year, during an execution at Oklahoma State Penitentiary, the condemned prisoner Clayton Lockett was administered an untested mixture of drugs that had not previously been used for executions in the US. He survived for 43 minutes before suffering a heart attack and being pronounced dead. During an earlier execution in Ohio in January, Dennis McGuire took more than 20 minutes to die and was gasping for air during the ordeal. Again, it was the first use of a new and untested use of lethal drugs.

Recently published research conducted by Professor Austin Sarat, of Amherst College, shows that the use of lethal injections is more likely to be botched than any other means of execution. Of the 1054 executions carried out before 2010, 7.1% were not effective compared to 3.1% from the 2721 who faced death by hanging or 1.9% from the 4374 who were executed by electrocution. The firing squad remains the form of execution least likely to be botched.

The use of lethal drugs was introduced in Oklahoma in the 1970s following public revulsion over killing by electric chair or by using a noose. The system consists of three drugs - an anaesthetic to render the prisoner unconsciousness and relieve pain, a paralytic to prevent movement, and a drug to stop the heart, causing death.

In recent years there has been a sharp decline in the number of executions in the US following its re-introduction in 1976. From a high of 99 in 2000, there were 39 in 2013, and this year Maryland and California became the latest US states to ban the use of the death penalty as unconstitutional and arbitrary. Only 32 states still maintain capital punishment. Welcoming the decisions, Amnesty International's US director Steven W Hawkins said that "the US cannot be a country that truly values human rights as long as it continues to execute prisoners".