On the island there is very little Malaria, only in the dirty sub-urbs from Stonetown area.
Incase you go to the mainland, into the tropical forest, don't hesitate to take your pills. Otherwise, don't worry too much. We have been here on the coast for a long time now and we did not use any pills, because on the coast the malaria mosquitoes are not common. If however you are in doubt, always check with your practitioner at home before you go to Zanzibar.