Coming back from "the dead" has left Superboy more than just a little ghostly. This results in him having a different reaction to kryptonite. Something he and Clark both learn of firsthand when facing off against Metallo.
While Superman as usual automatically starts to weaken in proximity to the cyborg's power source, Kon el on the other hand is not only unaffected but also feeling drawn to the glowing meteor rock. What follows is a complete blur.
One minute Metallo is gearing up for a devastating blow, the next one Superboy is ripping the kryptonite out of his chest without issue, and to Kal el's horror wolfing it down at a speed on par with the Flashes.
As if that wasn't unsettling enough, once he's done polishing the space rock off Superman has to stop him when he tries to take off in search of more.
Hours later the half kryptonian clone is holed up in the medbay, skin red and blotchy while simultaneously nursing a mild stomach ache. Despite being temporarily inconvenienced by the allergic reaction, Connor is exceptionally eager to get his hands on more of the green rocks.