It's the most famous crash diet in history ... the Master Cleanse ... but according to celebrity super-trainer Jillian Michaels ... it's dangerous for your body.

Michaels was out in Malibu this weekend, when we asked about rumors that Beyonce dropped the baby weight by going on a Master Cleanse ... a 10-day program where people are only allowed to consume tea and lemonade mixed with maple syrup and cayenne pepper.

"Don't do the Master Cleanse," Michaels told us bluntly ... "It's bad for you."

Back in 2006, Beyonce told Oprah she shed a TON of poundage by using the program for 14 days ... but now, Michaels insinuates Beyonce is more educated about the dangers of the Cleanse ... and HIGHLY DOUBTS the singer would ever do it again.

But fear not ... Michaels DID offer some healthy weight-loss advice that's sure to work every time ... so watch the clip ... and then get off your ass and work out.

"Biggest Loser" trainer Jillian Michaels believes slim is in -- but not everyone (in the TMZ newsroom) agrees and now she's sparked a major beef. Mmm ... did someone say beef?



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"Biggest Loser" star Jillian Michaels was just slapped with another multi-million dollar lawsuit over her weight loss products -- and this time, it's about a detox supplement that allegedly "might kill you."


The class action lawsuit, obtained by TMZ, was filed today in L.A. County Superior Court by a woman who only wants to be known by her initials -- R.D. -- due to her "very real fear of retaliation OR harassment by Defendants." 

In the lawsuit, R.D. claims Jillian's "Triple Process Total Body Detox & Cleanse" diet supplement contains a "potentially lethal combination of toxic ingredients" ... and it "might kill you."

In the suit, R.D. lays out what she considers to be the risks associated with various ingredients in Jillian's product, including:

-- Irish Moss Powder ... "causes gastrointestinal ulcers" and is "so toxic that it is the gel commonly applied to aircraft wings to dissolve ice"
-- Bearberry ... "known to cause nausea and vomiting"
-- Yarrow ... "a toxic lawn weed that causes dermatitis"
-- Chinese Rhubarb ... "a harsh laxative and dangerous diuretic that can cause sever dehydration and may cause irreversible liver damage"
-- Fenugreek Seed ... "interferes with digestion and causes both diarrhea and gas"

R.D. is suing Jillian and Thin Care International for more than $10 million for "actively and fraudulently" conspiring to hide the alleged dangers of their product.

If this story sounds familiar, it's not the first ... second ... or even the third time Jillian has been sued by people who claim her diet products are potentially toxic.

No comment from Jillian Michaels' camp.