Can my 11-lb. dog still take his old heartworm meds if the box says they are for dogs 2-10 lbs.?

take the old meds back to your vet. they should be able to switch them out for you for the proper dose. you never want to play around with giving too little heartworm medication. and as far as worrying about giving too much, you would have to give more than an entire box to cause problems, but not enough is just that. not enough. better to be safe than sorry. you could also double up on your prevention until you can exchange the product.