What word has five letters but is pronounced as only one letter?

I am a poison, but can be bought at the store. I can help a conversation, or make you speak no more. Many have been lost to me, and locked behind doors. When I am done with you, you might end up on all fours.

You break this when first time meeting someone

What is it that has no weight, can be seen by the naked eye, and if you put it in a barrel, it would make the barrel lighter?

When alone, I can be honest. When I'm with friends, I could've bought you fresh croissants back in the day. And no matter how many we are, always at our best with a dab of mustard.