Afflicted companions gradually lose loyalty over time unless the expedition regularly consumes whiskey. Consuming whisky while short on loyalty, however, will cause loyalty to rise, making maintenance of loyalty straightforward - so long as you have whiskey on hand.
Afflicted characters may also consume whiskey from your inventory when camping in nature - this triggers an event where they attempt to use the whiskey they stole to start merrymaking with the rest of your expedition, with the option to allow it to continue or to stop the party. Allowing it grants +10 sanity, with a chance that other expedition companions also become Alcoholics. Stopping the festivities will decrease the afflicted companion's loyalty or cause them to become angry with you.
The crowd at the end of each successful expedition may comment on any Alcoholics in your expedition, as they apparently reek openly of alcohol.