Unless it was an issue that was fixed in one of the recent hotfixes, you might need to check your mods/settings/etc. I have had little trouble getting effects from pills offered to my character (forced or willing) nor causing effects on NPCs to whom I fed pills. Forcing a pill on an NPC tends to work most of the time, while it seems to be fairly difficult to get them to accept an offered pill - depending on the NPC.
Testing it just now, I had to stop actively hypnotizing and riding an NPC for them to seem to properly accept the offered pill. Sometimes the pill would disappear without triggering the effect, as though they had knocked it away (without the appropriate text showing) rather than just ignoring it or something.
So, as far as I can tell, it seems that offering a pill to an NPC will be less likely to fail (and lose that pill) if you offer it to them while you are not actively fucking /hypnotizing them, etc..