The pharmacy was clearly negligent in giving your father the wrong medication.  Whether it is worth his while to pursue a claim depends on how seriously he was injured.  If he suffered only a short-term "sickness" that has fully resolved, it may not be worth pursuing because the costs of bringing a claim may outweigh the recovery.  If he suffered a permanent injury, it is definitely worth considering.I suggest you call a couple malpractice attorneys in your area for advice.  Do NOT throw away the prescription bottle.  It is very important that you keep this evidence.