Phthalates are an established carcinogenic class of chemicals that are added to a stunning array of products. They are routinely added to plastics to make them softer, and you can detect their presence by a sweet or vanilla-esque smell on dolls and toys, including sex toys.
The cheaper toys on the market are often hard plastic vibrators or dildos that have a softer plastic coating on the outside: this soft plastic coating, called "jelly" by retailers, contains phthalates.
Phthalates are included in a list of substances “reasonably anticipated to be a human carcinogen,” but the jury is still out on whether (or how much) exposure to these chemicals causes cancer, and you can bet that NIH isn't going to investigate how much, um, "exposure" you get by inserting one of these jelly toys into your hole(s). If you've graduated past your high school vibrator, go for something silicone. It's nonporous (which means it doesn't absorb bacteria), sturdy, and lasts forever. Plus you can get lots of pretty colors. An important note, however, is *not* to use silicone-based lubes with silicone toys, because they'll make them blow up. Or something bad.
Here are some silicone favorites: