Started cymbalta (duloxetine) today having a panic attack now is it safe to take a half a g of lorazepam to calm this down what should i do?

Yes: It will take a few weeks for Cymbalta (duloxetine) to help to prevent panic attacks because you need to develop a consistent blood level in your bloodstream. Lorazepam is a good rescue medicine is you have a break through panic attack. If the number of attacks are not decreasing over the next few weeks, you may need to switch to a different medication. Everyone responds differently. Discuss with your doctor.

Talk to your doc!: You & your doc should have a game plan in place to prevent panic attacks (that's cymbalta) and stop them (possibly lorazepam). You're probably referring to 0.5mg, not 0.5g, but regardless, this question needs to be addressed to/by your doc. In general, benzodiazepines can be used to stop panic attacks although dose varies from person-to-person. Be careful of risk for addiction, confusion & death.