Can you notice if a person misses one dosage of anti-depression medication?

My friend won't tell me all of the medication they are on, but it seems like they have bipolar disorder even though I don't think they are being medicated for that. The only thing I know for sure is that he takes medicine for depression, I'm not sure what. While a group of friends went away for the weekend, he seemed good until one day he was just being rude for no reason. He left his pills on the bathroom counter and although I noticed there were two left over in the travel size container. I didn't want to question him because he could have brought extra for some reason, but I was wondering if a missed dose could have almost immediate effects? Sorry if this is long, thanks for reading!