Mark 16:18 (NKJV), "they will take up serpents; and if they drink anything deadly, it will by no means hurt them..."
This Scripture in the book of Mark has often been misunderstood. So much so that some religions have even encouraged their followers to receive poisonous snake bites to demonstrate there faith in God. Nevertheless this is not what this verse means.
Mark is the second book of the New Testament. The New Testament was first written in Greek and later translated to English and many other languages. I want to bring your attention to the word that is translated to "drink". The original Greek word used here is a word pronounced "pinō". (Strong's Greek & Hebrew Dictionary) This word is more accurately defined as the word "imbibe".
In simpler terms the word imbibe can mean to drink something. However according to dictionary.com the deeper meaning of this word is to "absorb or assimilate ideas or knowledge".
The other word I want to draw you to is "serpents". Jesus said in Luke 10:19, "Behold, I give you the authority to trample on serpents and scorpions, and over all the power of the enemy, and nothing shall by any means hurt you." Serpents and scorpions are the demonic forces in the spirit realm.
The meaning of this verse is more correctly interpreted, "when the devil and his gang try to torment you in your thoughts it will not harm you". Backing up a couple of verses we see that this passage was written to, "He who believes and is baptized..." ( Mark 16:16)
The moment that you accept the gift of salvation you have stripped the enemy of any power over you. Just like an inheritance, if you don't know about it you can't benefit from it. The enemy will still try to deceive you though. He will try do this in your thoughts.
If your thoughts are tormenting you or keeping you up at night, you have a couple of options. The first one is usually the easiest. The Bible says in Philippians 4:8 (MSG) 8 Summing it all up, friends, I'd say you'll do best by filling your minds and meditating on things true, noble, reputable, authentic, compelling, gracious—the best, not the worst; the beautiful, not the ugly; things to praise, not things to curse.
In other words refuse to think about ugly things. Fix your thoughts on things you enjoy that make you happy. When our thinking is always on troubles we lose our joy. We go to bed at night and toss and turn rather than sleep. This is the goal of the enemy.
You need to take a break from these issues. God instructs us to. If you can't seem to change your thinking on your own then ask God to encourage you. Then sit down with your Bible or listen to a good sermon and you will be encouraged.
Just remember the only place old slew-foot (satan) can try to get you is in your thinking. Learn how to replace his lying thoughts with things you like to think about.