OPIOIDS

Opioids are the only clearly effective centrally-acting antitussive drugs and are thought to work by suppressing the brainstem cough center through mu and kappa opioid receptor agonism.
They are the first-line symptomatic treatment for severe, distressing cough.
All opioids used to treat cough have typical opioid side effects, such as sedation, constipation, and nausea.

Codeine: duration of action is 4 hours; usual adult dose is 10-20 mg every 4-6 hours. It has shown to be effective for acute and chronic cough; it is available alone or as an elixir with guaifenesin.
Dextromethorphan: duration of action 3-6 hours; usual adult dose is 10-20 mg every 4-6 hours. It is the most commonly used antitussive. It is as effective as codeine for cough. It is available alone or as an elixir with guaifenasin.
Hydrocodone: duration of action 4-6 hours; usual dose 5-10 mg every 4 hours. It has been shown to be as effective as codeine but with fewer gastrointestinal side effects; for this reason it is considered as the antitussive of choice.

All opioid analgesics have antitussive activity and there is no strong evidence that any one opioid has superior efficacy for cough. For patients already taking opioids for pain, it is unclear whether adding a second opioid, such as codeine, for cough is effective.

NON-OPIOIDS

Centrally-acting non-opioid antitussives
Gabapentin: the pathophysiology of refractory chronic cough is thought to resemble central sensitization as seen in neuropathic pain. Gabapentin can meaningfully improve cough-specific quality of life and reduce cough frequency and severity with doses up to 1800 mg a day.
Other agents: paroxetine, amitriptyline, and benzodiazepines have been anecdotally reported (without evidence) to have efficacy in chronic, refractory cough.

Peripherally-acting antitussives
Sweet syrups are commonly used as cough suppressants, whether as bases for prescription elixirs (such as codeine with guaifenesin) or home remedies (honey, simple syrup). The mechanism of action is unknown; it may act as a protective barrier to sensory receptors in the throat that heighten the cough reflex.
Benzonatate inhibits cough by anesthetizing stretch receptors in the respiratory tract. Its duration of action is 3-8 hours; dosed at 100-200 mg three times a day. Side effects are uncommon but include sedation, headache, bronchospasm, and nausea. Empirically many experts recommend adding it to an opioid.
Antihistamines and anticholinergics are often part of combination antitussive elixirs with or without an opioid. Anticholinergics such as hyoscyamine and scopolamine are most helpful in the setting of copious upper respiratory secretions leading to cough.
Expectorants thin bronchial secretions and ease expectoration. Examples include guaifenesin (200-400 mg every 4 hours) and nebulized acetylcysteine or hypertonic saline. Empirically, they have been recommended for severe, chronic, wet coughs. Because they may increase fluid in the respiratory tract, they are not recommended if the cough reflex is diminished.
Nebulized local anesthetics are thought to work by anesthetizing afferent receptors in the respiratory tract. Published regimens include lidocaine 2% solution, 5 mL nebulized every 6 hours; and bupivacaine 0.25%, 5 mL nebulized every 8 hours. Bronchospasm is a potential side effect.
Other agents such as bronchodilators and corticosteroids have not been shown to be effective apart from specific indications (e.g. for COPD or asthma exacerbations).

See reference for details.

Adapted from Marks S and Rosielle DA. Palliative Care Network of Wisconsin. Fast facts and concepts #199 200. Opioids for cough. Internet. January 21, 2016.