'Mom' Star Kristen Johnston Blames Lupus Medication For Shocking Weight Gain

Kristen Johnston returned to Mom this season to reprise her role as Allison Janney's troubled foster sister Tammy, but Johnston looks different this time around. The actress addressed her weight gain over Twitter and exclusively revealed to RadarOnline.com if fans could expect her on the show next season.

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Johnston, who suffers from Lupus, revealed she "gained a lot of weight due to the meds" she was taking for the disease.

Johnston exclusively told RadarOnline.com that she will be on "this season and next season!"

Johnston's weight has fluctuated over the years. In 2008, she opened up about losing over 60 pounds after going from a size 6 to 12. "After a show, you're so hopped up you have a couple of drinks with friends," she told People. "'Yeah, I'll stay for another round!' That, plus a diet of 'pizza, BLTs and fries, dinner at 11 p.m."

Johnston has been open about her battle with Lupus. She was diagnosed with the disease after visiting 17 doctors. She was treated with chemo, steroids and was in remission. She revealed that she couldn't lift her neck without a brace. "If I had to live like that forever, I would have killed myself," she told People. "Every single day is a gift, and I don't take one second of it for granted."