Guardian One® for Dogs 46-91lbs 13ct. This flea monthly prevention capsule protects medium to large dogs from infestations. Nitenpyram starts working in just 30 minutes and Lufenuron provides a full 30 days of protection. Both ingredients are considered the gold standard in the pet supply industry. Additionally, they are gentle and time-tested, making them safe for your dog. Plus, with no preservatives or artificial flavors, you can trust that Guardian only uses high-quality, all-natural ingredients. The airtight bottle cap and gelcaps locking design also ensure long-lasting freshness compared to other flea treatments.