Why does Billy Costigan only drink cranberry juice in the departed?

Answer 1

Some people believe that cranberry juice can mask the presence of drugs. Costigan is just out of jail and probably seeing a probation office. He's also selling cocaine with his cousin. He may be ordering the cranberry juice because, as an undercover agent, he wants the criminals he's hanging out with to think he is doing drugs and trying to mask the presence. Or maybe he really is doing drugs and needs to hide the fact.

At least that's my theory.

- Not Quite. He's still a federal agent, regardless of the conditions of where he is, he is still unable to drink or be high during the period in which he is working undercover. Simple law.

-It shows Billy Taking OxyContin (opiate painkiller) so he is probably playing it safe and not drinking, as mixing the two can easily be fatal