Seasonal Flu, H1N1 Medications – Side Effects, Adverse Reactions & Cautions

This post summarizes possible side-effects and cautions that are important to be aware of while using common antiviral agents both for the seasonal flu and a Novel H1N1 infection.  These awareness guidelines apply to both adults and children; and additionally, are important for pregnant women to bear in mind as well.

ZANAMIVIR:

The drug Zanamivir is licensed only for use in persons without underlying respiratory or cardiac disease.   Post marketing surveillance as well as some studies indicate that respiratory function deterioration can occur after inhalation of Zanamivir by those who have underlying airway disease.   For this reason, this drug is notrecommended for patients with such an underlying condition.  Post marketing surveillance has also included reports of allergic reactions such as oropharyngeal or facial edema in some cases with the use of this drug. 

Other common adverse events reported by those using Zanamivir include diarrhea, nausea, sinusitis, nasal signs and symptoms, bronchitis, cough, headache, dizziness, and ear, nose, and throat infections.   Each of these symptoms was reported by less than 5% of persons in the clinical treatment studies.

OSELTAMIVIR:

Relative to Zanamivir, a greater number of unpleasant side-effects appear to have been reported with the use of the antiviral medication, Oseltamivir.  In clinical treatment trials,  nausea and vomiting were reported more frequently among adults receiving Oseltamivir for treatment (nausea without vomiting, approximately 10%; vomiting, approximately 9%) than among persons receiving placebo (nausea without vomiting, approximately 6%; vomiting, approximately 3%).  Among children treated with Oseltamivir, 14% had vomiting, compared with 8.5% of placebo recipients.  It is recommended that Oseltamivir be taken with food to help reduce the severity of nausea and vomiting.

Another concern with respect to the use of Oseltamivir has been reports oftransient neuro-psychiatric events(such as self-injury or delirium) that appear to have been associated with its use.  Currently, the FDA has advised that persons taking Oseltamivir be monitored closely for abnormal behavior.

AMANTADINE & RIMANTADINE:

At a dosage of 200 mg/day, both Amantadine and Rimantadine can cause Central Nervous System (CNS) and gastro-intestinal side-effects when given to young, healthy adults.  Data suggest that  incidence of CNS side effects (e.g., nervousness, anxiety, insomnia, difficulty concentrating, and lightheadedness) is higheramong persons taking Amantadine than among those taking Rimantadine.  Generally, side effects associated with  both these drugs are mild and cease when the drug is discontinued.  However, serious side effects have been observed (e.g., marked behavioral changes, delirium, hallucinations, agitation, and seizures) among persons who have renal insufficiency, seizure disorders, or certain psychiatric disorders and also among older persons who have been taking Amantadine as prophylactic treatment at a dose of 200 mg/day.

DRUG INTERACTIONS:

Data vis-a-vis drug interactions is limited with respect to both Zanamivir and Oseltamivir.  With regard to the use of the drug Amantadine, caution is advised in using it concurrently with drugs that effect the CNS, including CNS stimulants.  The concomitant administration of antihistamines or anticholinergic drugs can also increase the incidence of adverse CNS reactions in patients.  Currently, no published data are available concerning the safety or efficacy of using combinations of any of these influenza antiviral drugs.  For this reason, awareness and cautions must always be borne in mind while using these medications.  Use this link to refer to helpful criteria for discerning an adverse drug event/interaction.

SAFETY ISSUES DURING PREGNANCY:

The four drugs discussed above fall under “Pregnancy Category C”medications, which indicates that no clinical studies have been conducted to assess the safety of these medications for pregnant women.  Both Amantadine and Rimantadine have been demonstrated in animal studies to be teratogenic (i.e., able to disturb the growth and development of an embryo or fetus) and embryotoxic when administered at substantially high doses.  CDC recommends that the antiviral medications discussed here should be used during pregnancy only if the potential benefit justifies the potential risk to the embryo or fetus, and further, that the manufacturers’ package inserts should be consulted prior to the use of these medications.

With an astute awareness on how to use drugs safely, a great many potentially serious adverse events can either be circumvented altogether or treated promptly.  If you are currently dealing with the seasonal flu or an H1N1 infection, stay on the alert while using prescribed medications, and use them according to the parameters provided.  You may also want to keep the above information handy or pass it on to others who may benefit.

ADDITIONAL USEFUL RESOURCES:

Prevention & Control of Influenza – Recommendations of the Advisory Committee on Immunization Practices (ACIP) 2008. MMWR 2008 Jul 17; Early Release:1-60.

Prevention & Control of Influenza – Recommendations of the Advisory Committee on Immunization Practices (ACIP) 2004. MMWR 2004 May 28; 53(RR06);1-40.