Safeguard and Valbazen are both good to use, the both kill the worms more slowly and there is less risk of overloading the system with dieing worms. I use both regularly. I have to worm for roundworm about every 3 months in my flock, every flock is different. Some people can do once or twice a year, some have to do even more often than I. I found my frequency by watching the birds, and droppings. I tend to start seeing runny droppings, and weight loss can be an early sign also, though you won't know unless you pick them up and feel them. Feathers hide a lot. With heavy loads of worms you can see slow and doughy crops, and sometimes complete crop stasis from what's going on in the gut. Early on I lost a couple of birds to secondary infections from roundworm, so I don't mess around with it any more. I've had some very sick birds from roundworm. I've been worming for years and there have been no negative effects in my flock, there were negative effects from not worming. I've never killed a bird by worming it. Again, every flock is different, every environment has a different worm load, so what works for you may not be the same as what works for someone else. I have a near neighbor that does not have the worm load I have, even though we live fairly near to each other. Once the bird has picked up the worms they can multiply rapidly, so you can go from zero to a heavy load rather quickly.