Many weight loss pills do have speed in them.
Whether they are dangerous or not & whether they work or not depends on how many you take (an overdose or just more than recommended) is too much & could cause serious problems for many body organs, even death as it can damage the heart; it also depends on what & how much you eat, having a healthy diet & some exercise along with the recommended dose of diet pills as a short term aid is probably fine & will work, but having no exercise & still eating lots of rich sweet food & the diet pills will be useless.
The other thing though is that we are all different, we can all have different reactions to any food or medication so even a low dose diet pill & the right eating plan could be dangerous if you are affected by a serious side effect.
Are they really that dangerous? This not only depends on what you're taking but how much, what you're eating, how much exercise you'er doing & whether the particular diet pill has any side effects to which you are susceptible!!