They are quite often cheaper than other brands, I’ve never had a problem with them. Just check the bottle isn’t too big- no point buying a cheaper, bigger bottle that will go out of date before you use it all. I try to buy wormer in 1 litre bottles to save wastage. Withdrawals differ because of how long they stay in the body, how strong the product is and how they are metabolised... it is what it is, no point looking too much into it just pick one that suits your system and the worms you’re trying to kill.