According to TV Line, Silver (Jessica Stroup) will begin to struggle with the condition again after Adrianna (Jessica Lowndes) finds out about her affair with Navid (Michael Steger).
Stroup explained: "Adrianna is not likeable now, and she's only going to continue to do worse things to Silver.
"The bipolar disorder is something she's been medicating herself to fight against. Unfortunately, someone tampers with [the medicine] and stuff's about to blow up!"
Sources on the show added that Silver's struggle with bipolar disorder will last for a number of episodes and could affect her romance with Navid.
90210 airs on Mondays at 8/7c on The CW in the US and on Tuesdays at 9pm on E4 in the UK.