The same regulations apply
Driving under the influence is driving under the influence of medications. This is the only real danger of medications generally, whether it is medical marijuana or any other legal medication. There is nothing dangerous about marijuana inherently, but for those side effects that could result in an accident, in which case, it is no more dangerous than any other prescription drug with similar side effects, particularly pain killers and muscle relaxers.
No, it isn't more dangerous.
Opponents of marijuana will argue legalizing cannabis for medical use would pave the way for other recreational, illegal drugs. However, prescription drugs can be quite addictive and produce more than enough side-effects. I have never heard of anyone overdosing on marijuana. Cannabis is a plant that grows naturally in the ground.