sentinel requires a prescription from your veterinarian. it’s also important to eliminate fleas and to not feed your pet rodents or uncooked meat or fish. if your pet is off sentinel for 6 months or less, re-start the medication and test after 6 months since microfilaria, that take 6 months to mature, will result in a positive test result. if a dose is missed and a 30-day interval between dosing is exceeded, administer sentinel immediately and resume the monthly dosing …