That's a good question. I never fed medicated so I didn't think about that. Adults are fine eating chick food, the biggest difference is chick food is higher in protein. As far as I know, the medication won't hurt older birds but hopefully someone will chime in with a more concrete answer. The other question is if any of your 18 week olds are laying? If any of them are, and you offer chick food to everyone, be sure you put out a dish of oyster shell for any layers. Chicks and non layers won't eat it and layers will take what they need.