I wouldn't worry too much about using Frontline Plus you get from your vet. We've been using it for years, on I'm guessing hundreds of pets and to my knowledge we've only had two issues (one was just a ticked off cat, not really a reaction, the other was some minor hair loss at the application site).
I can't say for the safety of OTC/internet Frontline as it's possible to have counterfit products.
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