Went to the er worst headache ever and blurry eyes got prescribed topimax how long till its effective?

Variable: It is an effective medication that may take a couple doses before being effective. Sometimes it is necessary to increase the dose over a period of a couple weeks.

Headache: It's hard to tell how long it may take before topamax (topiramate) will be effective. It works for migraine headaches but not for other types. It's a prophylactic medication and if you don't have frequent headaches (more than 3x/week), it may not be quite an option for you.

Diagnosis?: What is your diagnosis? If this is migraine you might get better in a few hours. If your headaches and blurry vision are from pseudotumor cerebri, hypertension, subarachnoid hemorrhage, etc the topamax (topiramate) will not help.