Vet just phoned - no cancer!

Infection caused lump - foreign body suspected, so we are assuming it was an insect bite - tic head possibly left in when it got brushed off. Just need to keep an eye on him for flare up of infection, or fluid build up. PHEW!
Advantixed about 6 weeks ago so it does what it says on the tin - only gives protection for a month! Here we go £30 every month from now on, or will I try Garlic as recommended by some of my doggy friends?
Could I give them garlic while the Advantix is still in their system or is that OD - ing?