LOSS OF CONTROL can be driven by physical or psychological factors - or both.

Physical dependence is when the body comes to need a drug to function normally. If it's not taken, unpleasant withdrawal symptoms occur. The only way to avoid this is to take more drug(s).
Psychological dependence is when an individual comes to rely on a drug to supply good feelings - such as relaxation, self-confidence, self esteem, freedom from anxiety etc.
This is not just a casual desire, it's a powerful compulsion.

So, here's a scenario:
A person first takes a drug out of curiosity, peer pressure, boredom - whatever.
They like what it does for them - it makes them feel self confident, relaxed, powerful.
So they take it again - to try and get these good feelings back.
And again . . . and again . . . and again . . .
Now they're becoming either physically or psychologically dependent on the drug.
    They have to keep on taking it just to feel normal. They've got no choice.

They've LOST CONTROL OF THEIR DRUG USE

Although they'll probably not be able to admit this - even to themselves.