Frontline and Advantage have been on the market for years and are about as safe as flea products can be IMO. The only problem is that since they have been around so long, fleas are starting to develop a resistance to them.
I prefer Interceptor to Heartguard, though. Heartguard is Ivermection which studies have shown can cause seizures in some dogs. The dosage is also more appropriate for our Maltese, under 10 pounds. Heartguard is under 25 pounds.