The most common side effects when taking Tamiflu is which is generally well tolerated in most people are nausea and vomiting however, taking Tamiflu with food may reduce the potential of these side effects.

Gilead Science trialed a total of 1171 patients and those who participated in adult phase III controlled clinical trials for the treatment of influenza were treated with Tamiflu. The most frequently reported adverse events in these studies were nausea and vomiting.

A total of 945 healthy young adults and 495 “at risk” patients such as elderly patients and patients with chronic cardiac or respiratory disease took part in clinical testing. These people were tested to see what effects the drug would have on people with other illnesses. Patients taking Tamiflu compared with placebo showed an increase in nausea, vomiting, bronchitis, insomnia, and vertigo. Bronchitis, insomnia and vertigo reported less frequency. It was seen that there were some common adverse drug reactions associated with Tamiflu therapy including nausea, vomiting, diarrhea, abdominal pain, and headache. Very rarely there were more serious adverse drug reactions; these include hepatitis and elevated liver enzymes, rash, allergic reactions including anaphylaxis, and Stevens-Johnson syndrome.

Some other adverse drug reactions have been reported and these include toxic epidermal necrolysis, cardiac arrhythmia, seizure, confusion, aggravation of diabetes, and haemorrhagic colitis.

Japan's health ministry in May 2004 ordered changes to the literature accompanying Tamiflu to add neurological and psychological disorders as possible adverse effects; these included impaired consciousness, abnormal behavior, and hallucinations. Various cases of psychological disorders have been associated with Tamiflu between 2000 and 2004, including several deaths.

There is concern in the possibility of an influenza pandemic, since resistance is more likely to develop due to the potentially longer duration of infection by new viruses. Genetic sequence for the neuraminidase enzyme is highly conserved across virus strains. By this we mean that there are relatively few variations, and there is also evidence that variations that do occur tend to be less "fit." Mutations that convey resistance to Tamiflu may also tend to destroy the virus by giving it an otherwise less-functional enzyme. Lack of variation in neuraminidase gives two advantages to oseltamivir and zanamavir; these are drugs that target that enzyme and they work on a broader spectrum of influenza strains making the development of a robust, resistant virus strain appears less likely.

I common with any prescribed drug, you should talk to your doctor before taking Tamiflu and make sure to tell your doctor about an other drugs you may be taking.