Didn't finish antibiotic course, please help? Watch
Depends how many days ago you were taking the medication and how frequently you were taking them - if you were taking 50-100 mg four times a day you've not really missed a large proportion of what you were prescribed so not to worry.
As ForestCat has advised, throw it away.
About a month or so ago I went to the doctors for a UTI infection and they gave me some nitrofurantonin tablets to take over 3 days. I thought I had finished them at the time but I've just been emptying my rucksack out and I've found I've missed one of the tablets because it fell out of the packet. Should I take it now or what?