Comfortis is a life saver here in Florida land of the sand fleas. [You know they can exist dormant for up to 3 years [!!!! until they find a host?] The trick that works with ours to prevent them throwing it up is be sure they have a full tummy first, wait about 30 minutes then give them the Comfortis. We also only give it every 6 weeks and have absolutely NO fleas on them or in house. Do not give them heartworm meds at least 2 days before or after either. Both are strong meds so their system needs adaption time.