The question above mainly applies to dramamine. I've only taken 1 tablet for carsickness, but I have read about experiences with that drug. I'm including other drugs that can make you hallucinate and think it's real.
Furthermore, why can you still see the hallucinations as reality if you affirm to yourself that the things you see aren't real?
In my opinion this has to do with how perception works in general.
There are many situations, where you "see" something, that is not there. Optical and other sensory illusions are a prime example. Usually these mis-perceptions result from sensory input that our organs of perception were not built to analyze. For example, the size-weight illusion stems from the fact that we don't have objects of equal size and equal outside material but differing weights in our natural habitat. You will easily recognize that all sensory illusions are induced by highly artificial stimuli such as straight and parallel lines, which have only existed in the most recent past and have as yet not had a big influence on human evolution.
In the case of drugs and hallucinations, your brain is stimulated in a way that is not meaningful for it, and it tries to make sense of what if "perceives" in the context of its evolution and experience. Obviously the stimulation by a certain drug comes close to what the stimulation would be like if outside reality were like you hallucinate it.
A drug induced hallucination is basically a sensory illusion, with the difference that you stimulate the brain directly, instead of the sensory organs.
To "break" an illusion, you need to provide a stimulus that overrides or "corrects" the misperception, e.g. a ruler to show that the lines are parallel (or not parallel) etc. Just knowing the truth (because you drew the lines yourself) doesn't change the effect.
Since drug induced hallucinations originate not in an outside sensory stimulus but in a direct manipulation of the brain, I would guess that touching the elephant head of your friend might not help to break the illusion -- you might even feel differently. But I have no experience with drugs and couldn't find any reports on this.
A lot of your perception / thought process and content are affected by Serotonin and Dopamine. Many drugs act on 5HT / Serotonin1 which augments the mechanisms / pathways which are hard wired to your thoughts and perception of reality. As of personal experience serotonin affects more of the content of thoughts whereas dopamine also affects the structure / flow of thoughts.
To illustrate this: "Serotonin is widely known for its effect on mood, but it also plays a role in memory, anger, fear, appetite, stress, addiction, sexual pleasure, sleep, pain [...]"2 which all have an effect on the content of your thoughts. As you all probably experienced in your life, a negative mood makes you inclined to negative thoughts, you all probably got angry because you were hungry.
Dopamine on the other hand is a neurotransmitter which plays a major role in psychosis3. Racing thoughts, paranoia and delusions also affect the content of thoughts but its structure / flow is augmented as well.
Both play a crucial role in affecting your thought processes which in turn are part of your perception of reality.
Your perception of reality is a very complex process and a multitude of factors play into it, but the structure and content of information that flows through your brain which is a mix of thoughts and sensory input which together make up much of your constructed reality in your brain. "The brain constructs representations of what is sensed and thought about in the form of nerve impulses that propagate in circuits and network assemblies (Circuit Impulse Patterns, CIPs). CIP representations of which humans are consciously aware occur in the context of a sense of self." 4
I know from personal experience that perception of reality is affected by 5HT / D2 which is probably why drugs like LSD / MDMA / THC can induce mental illnesses such as depersonalization / dissociation / paranoia / psychosis. Those are all mental states which are affected a lot by neúrotransmitters like serotonin and dopamine. 6
Drugs act on these receptors and mechanism and augment pathways which influence neural patterns which ultimately make up your construct of reality.
In some cases paranoid thoughts also may induce fear which makes you less likely to question it's integrity and accept it as reality because you're in fight and flight mode.
With experience you can distinguish hallucinations from reality but it's still difficult to e.g. tell paranoid thoughts apart from rational thoughts.
