Republican Vice-Presidential candidate Governor Sarah Palin's (R-AK) daughter Bristol attends the third session of the 2008 Republican National Convention in St. Paul, Minnesota with boyfriend Levi Johnston September 3, 2008. REUTERS/Rick Wilking

ANCHORAGE, Alaska (Reuters) - The mother of an 18-year-old man who plans to marry Alaska Gov. Sarah Palin’s pregnant daughter, Bristol, has been arrested on drug charges, authorities said on Friday.

Sherry Johnston, 42, was taken into custody at her home in Wasilla, Alaska, on Thursday after an undercover narcotics investigation, Alaska State Troopers said in a statement.

She faces six counts of misconduct involving a controlled substance, said trooper spokeswoman Megan Peters. She was released on $5,000 bail shortly after her arrest. All the charges are felonies.

Peters declined to release any further information about the case, saying police needed to protect the investigation.

Sarah Palin, the Republican vice presidential nominee in the 2008 election, revealed during the campaign her 18-year-old daughter was pregnant and that Levi Johnston was the father.

The two have said they plan to marry and Bristol Palin is due to give birth this weekend.