San Diego Fire-Rescue Department responded to a report of an explosion inside an apartment on the 2100 block of Grand Avenue around 6:45 p.m.
The explosion did not ignite a fire, SDFRD spokesman Maurice Luque said.
Both victims were in their 20s and transported to UCSD Medical Center. One victim suffered serious injuries and the other had minor injuries, Luque said.
Metro Arson Strike Team investigators believe a meth-producing operation caused the explosion, a San Diego police officer said.
It was unknown if anyone was charged in the incident.