If you simply suspect drug use it may not be enough grounds to evict unless you can get proof. If you are serving the Form N7 based on the disturbances you should also obtain the Form L2 and a Certificate of Service, these are the two forms you require to apply to the Landlord and Tenant Board after issuing the N7. The cost of filing an application with the Board is $190.00 in person and  normally if the Board decides in your favour they will order the tenant to pay you for those costs as well.