According to MedicineNet, homatropine and hydrocodone are used together in cough syrup because homatropine helps to counter and balance the negative effects caused by hydrocodone. Homatropine is also added in low doses to cough syrups containing hydrocodone to discourage deliberate overdoses of hydrocodone.
MedicineNet defines hydrocodone as an opioid narcotic pain reliever and cough suppressant. It is similar to other opioids, such as oxycodone, codeine, methadone, morphine and fentanyl. To suppress pain, hydrocodone and other opioids stimulate nerve receptors in the brain to raise the brain's threshold to pain or the amount of stimulation required to feel pain. They also lower the perception of pain.
MedicineNet states that nerves use a chemical called acetylcholine to communicate. Homatropine is added with hydrocodone to block the action of acetylcholine and is used to counteract the habit-forming properties of hydrocodone. However, homatropine can be dangerous for those with glaucoma because of its ability to increase pressure inside the eye.
MedicineNet also notes that hydrocodone comes with serious side effects such as dizziness, sedation, lightheadedness, nausea and vomiting. Other side effects such as drowsiness and constipation can also occur with the use of hydrocodone. It can impair thinking and the required physical abilities to drive a car or operate machinery. Hydrocodone should be used with caution for the elderly, debilitated patients and patients with serious lung disease because it can depress breathing. Since it can be excreted in breast milk, nursing mothers are warned to use hydrocodone cautiously.