IME it depends on what the alcohol is, and remember I'm a T1 diabetic, so on external insulin.
Based on the preference for the liver to detoxify alcohol before managing blood sugar by release of stored glycogen, responses are typically
Spirits, red/white wine (dry) - no immediate effect, later drop in blood sugar
Beer - immediate rise (2 pints can see me go from 5.0 to >8 depending on what beer), if you drink enough you may see a later drop
Mixers, alcopops, port etc - big spike, possible later drop.
I've seen that later BG drop on someone's continuous glucose meter trace and it can be very dramatic, which is why T1Ds are told not to dose fully for the carbs in their booze beyond the first one. Of course, the degree of the effect depends on dose.