The Singin' in the Rain actress was rushed to hospital over the weekend after suffering a bad reaction to medication, her representative told TMZ.
The 80-year-old was taken to Cedars-Sinai hospital "due to an adverse reaction she experienced to a medication she was currently taking".
However, it is currently not clear what medication caused the reaction.
Her representative added that the actress will be cancelling all her upcoming engagements over the next three months while she recovers.
Reynolds currently remains in hospital, but is reportedly showing signs of improvement.
The actress - the mother of Star Wars star Carrie Fisher - most recently starred in Katherine Heigl comedy One for the Money.