White oblong pill with the imprint S500 on one side and nothing on the other?

Headache medication

It’s acetaminophen 500 mg which is the same as Tylenol 500 mg

i just found your question because i was searching the exact same thing - couldn't find it either - then... went to my medicine bin in the bathroom - to see what i could find - it's 500mg acetaminophen - generic version i got from wal mart!!!

i was so concerned - that I actually needed to take if afterward to get rid of my headache!! Hope this helps!!

Googled it and this was found: KOS 500

Generic Name: niacin

Pill imprint KOS 500 has been identified as Niaspan ER 500 mg.

is used in the treatment of high cholesterol; niacin deficiency; hyperlipoproteinemia type v, elevated chylomicrons vldl; hyperlipoproteinemia; hyperlipoproteinemia type iv, elevated vldl (and more), and belongs to the drug classes miscellaneous antihyperlipidemic agents, vitamins. Risk cannot be ruled out during pregnancy.

500 mg is not subject to the Controlled Substances Act.

Hope this helps? Or please take it across to a pharmacy for identification, sorry could not be of much help!