Brand names

Adekin, Amanta, Amantagamma, Amantan, Amantrel, Amixx, Antadine, Antiflu-Des, Atarin, Atenegine, Cerebramed Endantadine, Infectoflu, Influ-A, Lysovir, Mantadine, Mantadix, Mantidan, Padiken, Symadine, Symmetrel, Viroifral and Virucid

Available as 100 mg tablets and oral solution

Background

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 that bind to receptors on nearby nerve fibers. In other words, one nerve sends a chemical message, and the other nerve captures and receives it. This is how nerves communicate with each other. The chemical message may be one of increased or decreased nerve/muscle activity depending on the type of chemical and type of nerve. Different nerve fibers carry different types of signals, ultimately sending instructions to muscles or carrying messages of perception.

Amantadine was first used as an antiviral medication against influenza, but it is mainly used now for its ability to inhibit something called an “NMDA receptor.” It is this receptor that is involved in the situation where someone who is anxious, stressed, or even angry might be more susceptible to physical pain than they would had been calm or unstressed. 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 the pain continues. (This latter phenomenon of pain persisting despite healing of the original wound is often termed “wind up.”)

The NMDA receptor is a nervous system receptor. When it is stimulated by neurotransmitters, it creates the ongoing sensation of chronic pain. Even worse, a stimulus that is normally not painful actually becomes painful. In short, stimulation of the NMDA receptor creates inappropriate pain, and shutting that receptor off would be very important in the management of pain, especially chronic pain.

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 a helpful addition to pain relief regimens for dogs and cats.

How this Medication is Used

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 complications. (Inadequate pain relief after surgery can create the wind-up phenomenon described above. The declaw surgery has received scrutiny for its potential to cause this phenomenon.)

Amantadine can be given with or without food. It is usually given once or twice daily.

If a dose is accidentally skipped, do not double up on the next dose, but give it when you remember and time the next dose accordingly.

Side Effects

Mild side effects include agitation, gas or diarrhea when first taking it. These should resolve with time. Amantadine also has what are called anticholinergic effects, which include dry mouth (which is seen as increased thirst), retaining urine, and an increased heart rate. These could be problematic when used with medications with similar effects or certain medical conditions.

Overdose in humans has resulted in heart problems, such as abnormal rhythm and high blood pressure, as well as seizures and respiratory distress.

Interactions with Other Drugs

Trimethoprim-sulfa (an antibiotic), quinidine (a heart medication), and thiazide diuretics may decrease the excretion of amantadine, yielding higher blood levels and 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, and 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.

Concerns and Cautions

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, caution is recommended 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.

Amantadine has a narrow therapeutic range, which means accurate dosing is important to avoid toxicity. The use of oral liquid formulations allows for more accurate dosing.

This medication is best used in conjunction with other pain relievers and typically takes 2-3 weeks of use before its effectiveness can be judged.