There is a risk of carcinogenic toxins entering your system if you use a straw with your hot drink unless you have a full plastic straw which does not melt, since plastic contains carcinogenic substances.
Like all plastic products, plastic straws contain chemicals. When heated, some of the chemicals are released into your drink which you will then injest. However, the chemicals are not highly concentrated enough to harm you.
prior to anaerobic exercise i used a plastic straw to stir a hot mocha. i then vommited (b4 n after shower) noticing what seemed like milk skin. i used hot water and realised i had drank melted plastic. - probz got brain damage cuz im feelin' kinda lame. awesome.