This typical polemical use of Mark 16: 17-18 was published first time by the Islamic propagation of Achma Deedat many years ago.
As ussually Muslims make assertions with out knowing the Scripture and its context, they throw out ridicolous ideas and no consideration is made on the analogy.
Mark 16:17-18 clearly states that certain signs will follow those who believe, and that is a fact; these signs are following the Christians, or at least those who are true and sincere Christians.
We are living in a era when Christianity is growing faster than ever before, a rough estimates states that 70.000 people convert to Christianity every day. Many of these are Muslims.
This revival includes the use of miracles and signs as well, I know a lot of Christians who have been used as tools in healing, and others who speak in tongues or possess the ability to prophecy, to look into your mind, etc.
I have ex-Muslim friends, who are now Christians, who became victims of their family after their conversion and were poisoned, and they survived without any damage.
Thus from a modern observation, these signs do follow those who believe.
However, and here comes my point. Muslims have attempted to misuse this passage to suit their own agenda, however, such attempts prove fruitless.
The passage speaks first of all about the community of believers, not a particular individual. If you look closer into the Scripture and study the doctrine of spiritual gifts, miracles and ministries, this will be clarified. Thus no where is it stated, that every Christian speaks in tounges or heals or prophecies. There is a great diversity of gifts and every true believer possess his own particular spiritual gifting.
As for the poison, the passage does not encourage Christians to deliberately drink poison, this is an act which not even Jesus Christ fell into, when Satan asked him to jump of a mountain to prove who he was. Then why should a Christian drink poison?
The passage thus points to incidents as with the one of my friend who was poisoned because of his conversion and survived.
Does this make any sense?