There is nothing you can do about this based on the mere fact that the tenant signed a lease with a no-pet clause as this cannot be enforced according to section 14 of the Residential Tenancies Act. However, if the pet causes damage, disturbances, allergic reactions or is of a breed that is inherently dangerous, you have grounds to issue a notice of early termination. Therefore, if you can prove that the dog causes you to have a serious allergic reaction and it is also causing damages to the rental unit, you can issue a Form N5. Once you serve this notice, the tenant has a seven-day period to correct the situation. If the problem is not corrected, you can then file an application with the Landlord and Tenant Board. Form N7 may be given instead of the N5 form where the landlord and the tenant live in the same building and the landlord has less than 3 residential units in the same residential complex.