OpenMRS contains prescriptions (MedicationPrescription) as a drug order. Dispensing (MedicationDispense) and administration (MedicationAdministration) would be up to a module. See this post.
There are several implementations that have made pharmacy and/or inventory modules. For example, we hope to hear about Bahmni’s use of OpenERP on next week’s Dev Forum. I’m not sure how many have created medication administration modules. You could google around github and/or ask Implementers.
Cheers,