I have a strong suspicion somebody is dosing(I'm not going to get into the exact drugs listed) people at a bar/restaurant I attend.
What is the best legal course of action that can be taken? What are the legal repercussions of dosing somebody?
Giving someone drugs without their knowledge or consent, say in food or drink, is a criminal act. At the least it is a form of assault, and possibly a more serious crime could be supported by the facts. Note that people's reactions to drugs vary, and serious harm or even death can result from drugs that do not have serious effects on most people. Very serious criminal charges might then result.
The facts should be reported to the police.
If this is a case where the people receiving the drugs know about them, and want them, that is a very different matter, although it may still be illegal depending on the nature of the drugs.
Note that under US law, an uninvolved witness is not normally required to report a crime, although reporting is strongly encouraged. This rule is different in different countries. That is, in some countries an ordinary citizen may be legally required to report a crime.
In at least one state any person is required to report a crime if a victim is in danger of bodily harm (Wisconsin statute 940.34) There may be similar provisions in the laws of other states.
People with some sort of duty of care, or who are made "mandated reporters" by statute, such as teachers and health professionals, may be legally required to file reports when they know or have reason to believe that a crime is underway or has taken place. Such statutes vary from state to state, and will be different in non-US countries.