Witness The Rolling
An essential rule to smoking marijuana is to make sure you see the blunt/joint get rolled up. Especially if you're smoking with a new group of people, you want to make sure that no one is adding anything extra into or onto the trees. Reality is, some people don't think marijuana gets them high enough so while they enjoy the weed high, they need a little extra kick to get them just right.
You don't want to find yourself in a situation where you feel higher than you want to feel; come to find out your weed was laced with PCP, or cocaine or any other narcotic that can alter your mind state. You should always be aware of what's going on around you. Never feel pressured into smoking something that you don't feel comfortable smoking. Always keep in mind that everyone has a different tolerance and just because someone you consider your "friend" can handle it, doesn't mean you can.
Marijuana is a friendship drug - you might have come across a good friend while smoking a blunt. However, if the person who's rolling decides to add any extra ingredients into the blunt/joint that you'll be hitting, you have the right to know. You shouldn't hang around people who find it funny or think it's cool to lace marijuana and watch amateurs confusion and tripping out over drugs they didn't know they consumed. Unfortunately, things like this do happen to people and if you live to tell the story, hopefully you don't make the same mistake twice.
To avoid the chances of you smoking something you didn't agree to smoking, always make sure you're present when the ganja is being rolled up. That way you see what's going into your blunt or joint and the choice is yours whether you want to partake or not. You may get used to having a trustworthy friend roll something and smoke with you however, not everyone can be trusted. Use your intuition when walking into a cyph or a situation where there are pre-rolls.