An apparatus for a controlled injection of a drug into a selected tissue of a subject by a practitioner, the apparatus comprising: a drive unit for supplying a drug at at least one flow rate; a handpiece for being held by the practitioner, the handpiece being connected to the drive unit for receiving the drug and being adapted for holding an injection needle to be inserted into the selected tissue of the subject by the practitioner, for passing the drug to the selected tissue at an instantaneous and absolute pressure that is at a tip of the needle, and that varies due to placement of the needle in the subject; a computer (102, 104, 106) connected to the drive unit and programmed for operating the drive unit to supply the drug at at least one flow rate; a pressure sensor connected to the computer (102, 104, 106) for sensing or calculating the instantaneous pressure on a real-time basis; a speaker connected to the computer (102,104,106), and characterized by the selected tissue being the Ligamentum Flavum, there being a minimum threshold and absolute pressure which is desired for administering the drug to the selected tissue; and by a feedback mechanism for supplying a perceptible signal to the practitioner that indicates when the instantaneous pressure has reached the minimum threshold pressure, for guiding movement by the placement of the needle at least until the minimum threshold pressure has been reached, the feedback mechanism comprising: the computer (102, 104, 106) being programmed for driving the speaker for supplying a perceptible auditory ascending signal that indicates when the instantaneous pressure is rising toward the minimum threshold pressure, the auditory ascending signal being for guiding movement of the placement of the tip of the needle at least until the minimum threshold pressure has been attained; the computer (102, 104, 106) being programmed for driving the speaker for supplying a perceptible auditory specific signal that indicates when the instantaneous pressure has reached the minimum threshold pressure; the computer (102, 104, 106) also being programmed for driving the speaker for supplying a perceptible auditory signal that indicates when the instantaneous pressure declines or fluctuates before reaching the minimum threshold pressure. a drive unit for supplying a drug at at least one flow rate; a handpiece for being held by the practitioner, the handpiece being connected to the drive unit for receiving the drug and being adapted for holding an injection needle to be inserted into the selected tissue of the subject by the practitioner, for passing the drug to the selected tissue at an instantaneous and absolute pressure that is at a tip of the needle, and that varies due to placement of the needle in the subject; a computer (102, 104, 106) connected to the drive unit and programmed for operating the drive unit to supply the drug at at least one flow rate; a pressure sensor connected to the computer (102, 104, 106) for sensing or calculating the instantaneous pressure on a real-time basis; a speaker connected to the computer (102,104,106), and characterized by the selected tissue being the Ligamentum Flavum, there being a minimum threshold and absolute pressure which is desired for administering the drug to the selected tissue; and by a feedback mechanism for supplying a perceptible signal to the practitioner that indicates when the instantaneous pressure has reached the minimum threshold pressure, for guiding movement by the placement of the needle at least until the minimum threshold pressure has been reached, the computer (102, 104, 106) being programmed for driving the speaker for supplying a perceptible auditory ascending signal that indicates when the instantaneous pressure is rising toward the minimum threshold pressure, the auditory ascending signal being for guiding movement of the placement of the tip of the needle at least until the minimum threshold pressure has been attained; the computer (102, 104, 106) being programmed for driving the speaker for supplying a perceptible auditory specific signal that indicates when the instantaneous pressure has reached the minimum threshold pressure; the computer (102, 104, 106) also being programmed for driving the speaker for supplying a perceptible auditory signal that indicates when the instantaneous pressure declines or fluctuates before reaching the minimum threshold pressure.