1. Probably yes
2.Buy the book, don't short change yourself here
3.Stick with Andreas or other reputable flush, they are mostly similar
I have a friend who was on psych drugs for depression for many years as a teenager. When she started flushing 10 years later, she did have a black out experience from the drugs being flushed out a few days after a flush. She was at a restaurant and had to lay down on the floor in the bathroom, it kinda traumatized her and she stopped flushing. Now she is ill again because her liver needs flushing and is worse than she started. Andreas warns against this, you can't start and stop in some cases, it will make you worse.
If it were me, I would go to one of the clinics like Narcanon and have them oversee the flush with me. This may or may not be an option, but most of us weren't angels and did drugs also, and flushed with no problems. In fact, getting those drugs out is a huge relief to the body.
Don't go into this without a guide, as in Andreas' book. There are many nuances covered you will need to understand.
Can you have someone around you when you flush the first few times to keep an eye on you? Can you stick around the house and have somebody "on call" in case you have some kind of scary experience?
I don't think my friend's experience is common at all, but they do have detox clinics for a reason.