VPT 311: General and Systemic Veterinary Pharmacology (2+1)

CENTRALLY ACTING ANTITUSSIVES

• The cough centre is located in the medulla oblongata, in close association with the respiratory centre.
• Specific neurons involved in cough coordination have shown to be suppressed by antitussives.
• Codeine (methyl morphine) is a naturally occurring alkaloid and has been the major antitussive
•  It shares many other actions of morphine at a less potent level, eg. it is analgesic and constipant.
• It can be converted to morphine in the body, but this does not give rise to habituation at the doses used for cough control, although addiction to codeine is possible.
• Codeine is well absorbed from the gut and owes its satisfactory duration of action to a slower metabolism.
• Codeine does not cause respiratory depression as does morphine.
• Codeine is effective at an oral dose of 1-2 mg/kg.
• Morphine and dimorphine (heroin) can be used as cough suppressant, their use is restricted to the control of severe cough in terminal stages.
• Butorphanol is a centrally acting opioid cough depressant that causes little sedation as compared to stronger opioid drugs.
• Hydrocodone is a narcotic drug that is used as an antitussive in which sedation is common and long-term administration results in constipation with an overdose of hydrocodone causing severe respiratory and cardiovascular depression.
• Non-narcotic antitussives have been developed to increase the safety of these agents for use in man, in an attempt to retain the ability of codeine to suppress the cough centre but to lose the CNS effects e.g. respiratory depression, analgesia and a tendency to habituation.
• Pholcodeine is about twice as potent as codeine, nosacapine (a histamine – releaser in dogs) and dextromethorphan are about equipotent, and dextrorphan is about half as potent.
• All share the mild sedative effect of codeine and its occasional tendency to induce vomiting.
• The levo isomers of dextrorphan and dextromethorphan have both analgesic and addictive properties.
• Oral route is conventional for cough remedies; these agents may be given parenterally.