An errant slider can go a long way.

Red Sox starting pitcher Ryan Dempster watched it happen three times in yesterday’s 12-4 pasting by the Toronto Blue Jays before 35,532 at Fenway Park.

The Blue Jays roughed up Dempster for six runs on seven hits that included three home runs over five innings. The three round-trippers were the most given up by a Red Sox pitcher this season.

“I didn’t make good pitches and I’ve got to do a better job,” Dempster said. “There were sliders that were left up in the zone and I just didn’t have the slider I normally have today, and that’s unfortunate.

“There were a couple of home runs, but I just didn’t execute on a couple of sliders. We’ll bounce back, regroup and go on the road and play really good baseball on the road.”

Dempster fell to 2-4 and his ERA puffed to 3.75 in 48 innings with 61 strikeouts and 19 walks. He had allowed six home runs in 43 innings before the Jays tagged him three times.

The Toronto batters didn’t stop with Dempster. The Blue Jays piled on another half-dozen runs and two more homers against the Red Sox bullpen.

“He made some good pitches but I felt like the game plan changed a little bit today from the last few games,” Sox catcher Jarrod Saltalamacchia said. “They didn’t miss any mistakes, that’s for sure, and it wasn’t just him (Dempster); it was the guys that came in after him.