A disposable drug delivery device has a dispensing needle, a plunger, a spring, a shaft, and a lock mechanism. The plunger is engaged with the inner surface of the dispensing needle, is capable of sliding in the cavity of the dispensing needle, and is fluidly sealed to the inner surface of the dispensing needle. The spring is connected to the plunger via a shaft. The lock mechanism holds the shaft in a first position such that the spring is in a compressed state. When the lock mechanism releases the shaft, the spring extends providing a force that moves the shaft and connected plunger in a direction toward the tapered end of the dispensing needle thereby expelling the drug from the dispensing needle into an eye. The spring also provides a retaining force so that the plunger is retained in an extended position against the tapered end of the dispensing needle.