The drug scene exists at Iowa State, but for students who have no interest in it, it is not very visible. Drugs are accessible to those who want them, but it is rare to see them being used openly. If you are into drugs, you will find friends who can help to hook you up, but the rest don't need to worry about drug pushers or usage around them. Students keep their habits private, with the exception of alcohol, and there is little threat on campus.
If you like large universities but are worried about drugs being prominent, then ISU may be a good choice. Drugs are obtained by having connected friends, so if you don't wish to partake, chances are that your friends will not either, and you won't have to worry about it.