The weedkiller manufactured by Monsanto known as RoundUp has been proven by different regulatory boards to control the growth of weeds than any other product on the market. This efficiency is why Monsanto was bought by Bayer about a year and a half ago.
Even though the product works great on weeds in your garden, RoundUp has a side effect that occurs when used. Whoever uses RoundUp eventually obtains cancer. Personally, I would rather have weeds in my garden than have cancer.