Wow. You fucked up. There is no good way to get out of this situation. Your lack of morals is sickening to me. I know people make mistakes, but willfully druging your husband is pretty fucking low. I don't believe in karma or hell, just that people have to live with with the consequences of their actions. Your actions are going to have some pretty serious consequences.

I cannot comment on the religious side as I am not of the same religion... it seems to me that while this man is blackmailing you to continue and you worry about him telling your husband... MY worry would be that you are doing real damage to your husband's health and you could even kill him.. Compared to that you confessing you had an affair might seem the lesser of two evils? To have an affair in your country is wrong I understand but I am thinking damaging his health or even killing him is worse?
I think you have to confess.. you have to stop poisoning him... and you have to tell your bf NO... enough.. no more.. finished..Your choice is to tell your husband yourself or to hope that the bf doesn't tell him..

Just to say that yes husband's do forgive .... many of them, especially if he knows you tried to end it and were blackmailed.. I can only hope he forgives you for poisoning him and damaging his health.

The consequences of your cheating are not the same as trying to cleanse yourself of the guilt that you are carrying personally for cheating. The consequences after you have revealed all to your husband shall come later. Whether good or bad. You cannot allow someone to continue to blackmail you. Good luck.