(for veterinary information only)
The nervous system involves millions of branching nerve fibers connecting and communicating with one another. The nerve ending of one fiber releases chemicals called "neurotransmitters" which bind to receptors on nearby nerve fibers creating stimulation or inhibition. Different nerve fibers carry different types of signals ultimately sending instructions to muscles or carrying messages of perception.
Connection (more technically called a
"synapse") between two nerve fibers.
Granules containing neurotransmitters
are shown as round blue-ish dots.
(original graphic by marvistavet.com)
Amantadine was first used as an antiviral medication against influenza but it is mainly used is now for its ability to inhibit something called an “NMDA receptor.” The NMDA receptor is a nervous system receptor which can bind to neurotransmitters such as aspartate (or more specifically to “N-methyl-D-aspartate”) or glutamate to create the sensation of chronic pain. Even worse, when glutamate or aspartate binds to this receptor, a stimulus which is normally not painful actually becomes painful. In short, stimulation of the NMDA receptor creates pain and shutting that receptor off would be very important in the management of chronic pain.
To put this in more common terms, it is this receptor which is involved in the situation where someone who is anxious, stressed, or even angry might be more susceptible to physical pain. Muscle cramps hurt worse, simple irritations are more noticeable. Everything just hurts more when experienced in the context of certain mental states. Another example might be persistent pain that continues long after the original painful wound has healed. There should not be persistent pain if one simply considers the present state of the wound yet pain continues. (This latter phenomenon is often termed “wind up.”)
When the NMDA receptor is antagonized or blocked by a medication such as amantadine chronic pain may be alleviated. Amantadine alone is not an effective analgesic but when combined with other pain relievers, it adds an extra dimension of pain relief. At this time veterinary experience with this drug is rather limited but it seems to be emerging as helpful addition to pain relief regimens for dogs and cats.
Typical situations where amantadine might be used:
- Treatment of arthritis pain.
- Treatment of neurologic pain (disk disease etc.).
- Treatment of pain associated with cancer, especially osteosarcoma.
- Treatment of chronic pain associated with declaw surgery (inadequate pain relief after surgery can create the “wind up” phenomenon described above).
Diarrhea, flatulence, and agitation have been described. Humans have reported hallucinations, dizziness, and other forms of mental side effects. Overdose (in humans) has resulted in heart problems (abnormal rhythm and high blood pressure) as well as seizures and respiratory distress.
Trimethoprim/Sulfa (an antibiotic), quinidine (a heart medication), thiazide diuretics may decrease the excretion of amantadine, yielding higher blood levels, making amantadine stronger.
Other anticholinergic drugs, such as antihistamines, may enhance the anticholinergic effects of amantadine. These effects include dry mouth (possibly showing as more lip-licking or water consumption), difficulty urinating, increased heart rate.
Other drugs that increase activity/general stimulation may exacerbate the agitation side effect sometimes seen with amantadine. Selegiline, which is frequently used to treat senile cognitive dysfunction might be an unexpected member of this category of drugs.
Probably the biggest concern is that this medication is fairly new to veterinary use and a full catalog of what to expect with its use is not yet available. Right now amantadine is growing in popularity for pets and may prove to be an excellent complement to many pain relief regimens.
In human medicine it is recommended to have caution when considering its use in patients with kidney disease, seizure disorders, active psychoses, liver disease, or congestive heart failure. This means that side effects are more likely in these situations.
Page posted: 4/16/08
Page last updated: 3/29/2015