Overdose - What is brown sugar heroin cut with?

[firefoxcub]

I had a friend, long time ago, who was a huge brown sugar junkie in India. He used to smoke it... that is he used to chase it on aluminum foil. One day, he showed me the brown power and asked if I could see some white crystalline substance in it. I could. He implied that this might be dangerous to smoke but he did anyway. He smoked about ten twelve lines before he passed out. In a while, he started to OD and i called an ambulance.
Now my question is... what could this white crystalline substance have been that would cause him to OD?

Oh and my friend survived after being in an 8 hour coma. He's clean now.