As soon as you can pick up the pills in the office and add them to your inventory, go into your inventory and just consume as many as you can. Once you reach 100% sanity you'll probably still have two pills left, so try to use one more to unlock the achievement.
Note: While you can pick the pills up to inspect them early on, you won't be able to add them to your inventory immediately. You'll have to wait until later in the story, for when the character realizes he needs to take his meds.