Im taking terconazole vaginal cream 0.4% & upon inserting an applicatorful it immediately started to ooz out, will it still be effective?

Probably: I'm assuming this is to treat a yeast infection? It is typical for some of the cream to leak out. It's always best for you to insert the medication at night, so that you're lying in bed for the next few hours while it works, and there's minimal leakage. If you finish the course of medicine, however, and are still having symptoms, talk to your doctor. Best wishes.

Yes: You should lie down to use the applicator and give it a few minutes then get up to wash your hands thoroughly. Preferably use before bedtime