How common is it for hasj to be mixed with opium in Holland?

Opium isn't typically "smoked" in the same way weed or hash can be. Opium works best when vaporized. If you were to try to burn and smoke it through a joint/bong/weed pipe, it's debatable how well it would work, but it would be very wasteful at least.

Opium is pretty rare and more expensive than weed/hash, as far as I know. I don't see a reason why coffee shops would give their customers free opium; especially when the customers wouldn't be aware of it or consume it effectively.

https://en.wikipedia.org/wiki/Opium#Consumption

https://en.wikipedia.org/wiki/Opium_pipe

There do seem to be some accounts of this online though. This article seems to suggest it's pretty common, but after a quick read through it's pretty misinformed on some stuff:

http://www.hiptravelguide.com/guides/modules.php?name=News&file=article&id=309&site=1

Maybe it is available, but I'm pretty sure you'd be told by the seller what exactly you were buying and you'd be paying substantially more.

http://bluelight.org/vb/threads/452014-Opiated-Hash-How-common-is-it

/r/Drugs Thread