In 1853, he invented the first hypodermic needle that used a true syringe and hollow needle. His biographer and brother-in-law, Rev Thomas Brown (1811-1893), wrote that Wood had taken the sting of the bee as his model. Brown also wrote, 'At first this new hypodermic method was employed exclusively for the administration of morphia and preparations of opium, but it is important to note that, from the outset, Dr Wood pointed to a far wider application.' In referring to the preface of a paper on '"New Method of Treating Neuralgia by Subcutaneous Injection," separately published in 1855', Brown quotes Wood as saying, 'In all probability, what is true in regard to narcotics would be found to be equally true in regard to other classes of remedies.'
There is a story in circulation that Wood's wife, Rebecca Massey, was the first known intravenous morphine addict and died of an overdose delivered by her husband's invention, however, Richard Davenport-Hines says, 'It is a myth: she outlived him, and survived until 1894.'
Wood is buried with his wife in Dean Cemetery in Edinburgh. The grave lies on an east-facing section of the obscured southern terrace. The gravestone corroborates the date of his wife's death.