Why does alcohol interact with medicine?

In they are: broken down using the same pathways - it may strengthen effects or side effects.

Liver enzymes: Alcohol can cause the enzymes in your liver (chemical which process medications) to act faster or slower which means the amount of medication in your blood may be unpredictable. Most antibiotics are less effective in the presence of alcohol. Some can make you very sick e.g. metronidazole. Alcohol enhances the sedative effect of medicines like strong pain killers, sleeping tablets, mood stabilisers