Are there any serious side effects or allergies to the glucose drink used in prescreening gestational diabetes?

I'm: Not sure but by now there would have many issues wit it if that was the case. It's likely just a test to measure your ability to handle increase blood sugar levels.

No Side Effects: It is just a drink loaded with sugar and what it will do is get absorped into your blood stream and raise your blood sugar.It will react differently in people with diabetese, in your case gestational diabetese.And in people without diabetese it will be different one thing is common it will raise blood sugar in both cases but there is no risk of allergies.