Welch, who played pub landlady Natalie Barnes, described leaving the set to score from a dealer and admitted driving under the influence of drugs.
The 51-year-old Loose Women panellist made the confession in her autobiography, Pulling Myself Together, which is being serialised in the Daily Mirror.
Welch said she began taking cocaine in nightclubs in her 20s but went on to become a regular user while suffering depression after the birth of her eldest son in 1989.
She said: “I’d reached a point where I felt taking cocaine was the only way I could survive.
“I was suffering crippling depression and I’d made myself believe coke was the only thing that could make me cope.
“In fact I was locked in a vicious circle because the more I took the worse the comedowns would be. I was sinking deeper and deeper.”
Welch, who went on to star in Waterloo Road, said she kicked the habit when she became pregnant with her second son Louis, who is now nine.