There is a slight difference here in the narratives. St. Matthew (Matthew 27:49) says, "And the rest said, Let be; let us see whether Elijah cometh to save him." Here in St. Mark the words are recorded as having been spoken by him alone who offered our Lord the vinegar.
According to St. John (John 21:1-25:28), the offering of the vinegar followed immediately upon the words of our Lord, "I thirst." This drink was not the stupefying potion given to criminals before their crucifixion, to lull the sense of pain, but the sour wine, the ordinary drink of the soldiers, called posen.
The reed was most probably the long stalk of the hyssop plant. Dr. J. Forbes Royle, in an elaborate article on the subject, quoted in Smith's 'Dictionary of the Bible', arrives at the conclusion that the hyssop is none other than the caper plant, the Arabic name of which, asuf, bears a strong resemblance to the Hebrew.
The plant is the Capparis spinosa of Linnaeus. The apparent difference between the narratives of St. Matthew and St. Mark may be reconciled by weaving in the narrative of St. John with those of the synoptists—the "Let be" of the soldiers in the one case being intended to restrain the individual from offering the wine; and the "Let be" of the individual, corresponding to our "Wait a moment," while he answered our Savior's cry, "I thirst."