1) Some beta-blockers (usually prescribed for heart conditions) can lessen the benefits. There is no way to determine how an individual brain will respond to these medications.
2) Any substance with a psychoactive component, it is likely to dampen the training outcome. The definition for a Psychoactive component is ďa chemical substance that acts primarily upon the central nervous system where it alters brain function, resulting in temporary changes in perception, mood, consciousness and behavior" - Wikipedia
3) Long time users of some pain killers are particularly difficult to work with at times, but not impossible.
4) Some anti-Anxiety/relaxant medications can be very difficult to train through. For example, Xanax is one we have had little success training through in our office.
You should be aware of the fact that some medications will often soften the training results. Thus, you are required to go through more sessions and longer recovery time.