First, anti-depressants can be prescribed for the treatment of certain anxiety disorders... so your friend might want to confirm that she's being given an anxiolytic (or anti-anxiety medication) and not an anti-depressant (in case you're curious, anti-depressants may be prescribed first because their side effects are typically less severe than the side effects for anxiolytics... and since anxiety and depression frequently go hand in hand, anti-depressants can relieve symptoms for both conditions).

Second, some medication is meant to be taken on a regular basis, whereas other medication is only meant to be taken when symptoms become unmanageable. For example, if your friend struggles with panic attacks, then she may only need to take the medication when she begins to experience a panic attack. For other anxiety disorders, she may be instructed to take the medication at regular intervals, and not just in "crisis" situations.

If your friend has questions about what she's being prescribed (which is perfectly understandable), then she really should talk to her doctor. I hope that doesn't seem dismissive, because I'm sure it'll be helpful to receive input from TeenHelp's members as well; however, because there are so many things that factor into making a decision about which medication(s) to prescribe, your friend's best bet for getting a comprehensive answer is to talk to her doctor.