Sometimes if you hold them for a bit, they'll swallow it. Believe me, I too hate dealing with rats that would rather drool out their medication then swallow it

Another thing that sometimes works is, after you syringe the medication into their mouth, wipe something yummy on their lips (baby food, chocolate, etc) just make sure it's liquid so they have to swallow what's in their mouth - otherwise they'll just snatch what you're offering and continue to not gulp down the medication.