Tamiflu (Oseltamivir) and Relenza (Zanamivir) are the two antiviral medications that are effective for the A-H1N1/09 virus. The Centers for Disease Control and Prevention information on the side effects of these medicines is:

Tamiflu

The side effects reported most often in those people who took this drug were gastrointestinal (i.e., nausea and vomiting). Nausea and vomiting may be less severe if oseltamivir is taken with food.

Relenza

This drug is inhaled and can cause side effects, especially in those with asthma or other chronic lung disease. Decreased respiratory function and bronchospasm have been reported with use of zanamivir. Zanamivir is generally not recommended for use in persons with underlying lung disease such as asthma and chronic obstructive pulmonary disease. Other side effects reported by less than 5% of those who have used this drug are diarrhea, nausea, sinusitis, nasal infections, bronchitis, cough, headache, and dizziness.

[More on side effects of these anti-viral medications:]

Reported side effects of oseltamivir are nausea and vomiting. Among children treated with oseltamivir in clinical studies, 14% had vomiting, compared with 8.5% of children getting a "placebo." (A placebo is an inactive substance that looks the same as, and is given in the same way as, a drug in a clinical trial.) Nausea and vomiting might be less severe if oseltamivir is taken with food. In addition, there have been reports of self-injury or delirium among persons with influenza who take oseltamivir. Most of these reports have been in teenagers from Japan, and it is still not certain whether oseltamivir or the influenza virus was responsible for these behavior changes. The U.S. Food and Drug Administration advises that people taking either of the two medications recommended for influenza (oseltamivir and zanamivir) be monitored closely for abnormal behavior.

Side effects from zanamivir have been reported in fewer than 5% of people participating in clinical trials and have been reported at the same rate in people receiving zanamivir as those being given a "placebo." Reported side effects have included diarrhea, nausea, sinusitis, nasal signs and symptoms, bronchitis, cough, headache, dizziness, and ear, nose, and throat infections. Also, zanamivir should not be used in people with underlying respiratory disease, including asthma.