Jbug has a good point. Sometimes anti-depressants will send someone with bipolar into a manic episode.

http://psychcentral.com/disorders/sx9.htm

Lexapro
Possible Side Effects

dry mouth
tremor or muscle spasms
nervousness
trouble sleeping
headache, drowsiness
nausea
drowsiness
insomnia
headache
increased sweating
dizziness, lightheadedness
changes in sexual function
Unreported side effects are possible, talk to your healthcare provider if any other symptoms occur.

Warnings/Precautions
This medicine may cause drowsiness or dizziness.
Alcoholic beverages can increase the effects of this medicine and should be avoided.
Smoking cigarettes can decrease the effectiveness of this medicine.
If you are planning to have surgery, let your doctor know.
Do NOT take this medicine if you have taken an MAO inhibitor [phenelzine (Nardil) or tranylcypromine (Parnate)] in the last two weeks.
This medicine can increase the risk of suicidal thinking and behavior in some children and teens. Parents of children taking antidepressants should watch for signs of worsening depression or unusual changes in behavior.

http://psychcentral.com/meds/lexapro.html#effects