Sometimes the mistake is with the pharmacy and other times it is with the doctor or his office.  Figuring this out can often be puzzling and it is essential that you obtain an attorney who is experienced in such matters.

The mere fact that you have had a terrible reaction to a prescription does not guarantee a successful claim.  Many modern day medications have known side-effects.  Some of these side-effects only occur in a very small group of people.  Analyzing such a claim requires an attorney sophisticated in such matters. 

You may or may not have a claim.  An analysis of this question will require a focus on the circumstances under which you were prescribed the medication, the dosage you were prescribed, whether or not you have other prescriptions or health factors which would make this prescription ill advised and a host of other considerations.  It is important that you consult a lawyer experienced in such cases so that a proper analysis to present your claim can be made.