Drake, some of the downsides of Opiates is that they suppress your appetite, and a lot of Heroin addicts suffer from malnutrition.
Some people get nauseous from Opiates, and some people are allergic to them.
And if you are buying Heroin from people off the street, you never know how pure it is, or how concentrated it is.
And I don't think I need to discuss the dangers of needles/syringes.
And always be careful when taking Opiate pills such as Percocete, Vicodin, etc… Because they contain a second analgesic called Acetaminophen (Tylenol), which in large doses can cause liver failure or liver damage.

But if you just smoke a little Opium or take a couple of pills, you probably won't have any problems... Just always, start off with the recommend dose for pain relief until you know how you will react.