Toxic is generally referring to having the effect of poison. It is also used in financial terms; debts that won't be repaid or things for which there is no value. It implies there's no return on what was put into it. And these all are very good descriptions for "friendships" we'd be wise to stay away from!
Oh how I love this. Never too late to live a life of goodness. I can change and I am going to. Only do positive things and good things for people. Never speak negatively. Never hurt anyone for personal gain. I will do better because, now, I know better.