A little pharmacology
Atropine is an Anticholinergic drug. This means that it inhibits the action of acetylcholine, a neurotransmitter both in the brain and the body. It has the distinction of being the transmitter that is used in sending signals to muscles.
Because of this, Atropine is used as a remedy for nerve gas exposure. Nerve Agents usually work by destruction of the uptake mechanism for acetylcholine. This causes the muscles to spasm out of control as acetylcholine builds up in the synapses. Atropine lessens the effect so that the body can fix the problem caused by the nerve agent.
Atropine, in higher doses then those used to dilate pupils, or medicinally (to treat some heart conditions) for other purposes, can cause vivid hallucinations where the user cannot tell the difference between hallucination and reality.
The first signs of effect from atropine are dry mouth and rapid heartbeat.