I would assume the consequences are
1. They lose their vote on the next issue. So that leaves the party less likely to be able to pass legislation.
2. They are penalised financially, either a fine or that week/month salary
3. They go on a public register. This could then be used to assess their character by the electorate come next election cycle.
Failing that you could simply criminalise drinking or drug decision making in the same way it is for driving.