What is a word for something that you always have?

[Nobody ❤]

"For a child milk shouldn't be a luxury but a necessity"

But, milk is already a necessary part of your diet! :| I know it's the opposite of 'luxury' but I feel like a different word belongs here, instead of 'necessity'. It's on the tip of my tongue and super annoying! I can't concentrate on anything right now but trying to think of this word. I can't eat. I can't sleep. I can't study. Please end my suffering and save my education.

What is a word for something that you always have and can take for granted?

HELP

p.s. If you're wondering where this slogan is from, I found it on this fb page:
http://www.facebook.com/MarigoldHLMilkSG