
Advocate is the only wormer licensed for the prevention of lungworm angiostrongylus vasorum - the lungworm advertised on tv and in vets surgeries and the one to be most concerned about. It must be used monthly if you live in a lungworm area and/or there have been definite infections or deaths in your area.
Worming to prevent this lungworm is a whole different ball game to the usual worming regime - it has to be done every single month without fail. It does seem that the younger the dog, the more likely they are to die from this, most probably because of immature immune system and more likely to investigate/lick/chew or pick up stuff that has been 'slimed' or the slugs and snails themselves. Toys, water/food bowls etc shouldn't be left in the garden. Poo should be picked up regularly and a nightly slug patrol should help too!
Your vet is wrong to give milbemax to your breed, some can be fine with it and some won't ( I think there is now a blood test to see if a dog carries the gene involved) and he shouldn't dismiss the adverse reactions to previous wormers - in fact they should report it to the manufacturers. If you have lost faith in your vet, you should really think about changing to one that is prepared to discuss the options and reasons to use one wormer over another.
Panacur has been given for my girl as she has started to react to Advocate after a year of monthly use- reactions of very laboured breathing, lethargy, coughing. The reaction time fell from a week to 12 hrs after dosing over a 3 month period - I didn't put two and two together until the 2nd month, did it once more to confirm! My vet confirmed my worries and changed wormer ( we had taken Eve to the vet with the 1st reaction, not realising then what or why it was happening - she was given tablets to help her breathing) She is now wormed monthly with Panacur as a preventative. Once a year she will be wormed on the 'treatment' dose over a period of 7 days, more often if she developes excercise intolerance, extra coughing etc though I will most likely have fecal samples taken. She is milbemaxed twice yearly.
My boy is still on Advocate with no problems and is milbemaxed twice yearly.
Some may think this is an over the top worming regime, but, having lost my young boy last year to lungworm having missed 1 months treatment, holding him while his lungs filled with blood and he bled from his nose and mouth, crying into his fur as he was pts - I will do all I can to prevent another death .
Read all you can on lungworm, find out if your area is affected, speak to other collie owners in your situation, and try to keep your garden as slug unfriendly as possible - clear leaves, poo and places they may hole up for the day! :-)