pertussis
"Taking 500 mg twice a day for pertussis(whooping cough). While I do feel some relief from the pertussis, there are some nasty side effects to it.

Firstly. Horrible GI symptoms. The medication causes a horrible taste for several hours after taking it, and it was causing abdominal pain and a feeling of too much acid in my stomache and an uncomfortable pressure only relieved by vomitting. The GI symptoms were unbearable, but I found eating cheese immediately before swallowing the pill helpped immensly. I also started a probiotic and prilosec for the stomache acid. 

Another issue was extreme muscle weakness (knees buckling) and loss of balance and coordination. I'm 21 with an athletic build and never had this issue before."