If they lost weight it's because they were too stoned to eat, aside from having the jitters associated with this drug.
In plain English, this is is how cocaine works: cocaine concentrates the blood into a much smaller space than usual [constricts peripheral blood flow], and increases the speed at which it [blood] moves through this smaller space. Hyperventilation also occurs.
When you breathe very fast [hyperventilate] your lungs' ability to take up oxygen into the blood actually decreases significantly. To make things worse, since cocaine damages the mucous membrane lining of the nose, lungs, etc.*, this means that the body is even less able to get oxygen into the blood.
Result: Too little oxygen - you die; too much blood pressure in the brain or heart - you die. This is not a joke; even first-time users can, and do, die.
The best help you can give your friends is to encourage them to seek treatment.
* That's why the blood-shot eyes, nose bleeds and coughing up blood.
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cocaine 
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