Monday, March 14, 2005

Tamiflu storage and adverse reactions

In response to my earlier post on the mechanism of action and brief comments on efficacy of the influenza A antiviral oseltamivir (Tamiflu), some readers had questions about storage. In addition, one reader (Gaudia) was kind enough to send links to scanned images of the drug as marketed in both the US and Canada. Those links (from the Comments) will be repeated at the end of this post.

With respect to storage, Gaudia sends this in: Tamiflu box says, STORE AT 25 DEGC (77 DEG F). EXCURSIONS PERMITTED TO 15 DEGTO 30 DEG C (59 DEG TO 86 DEG F). SEE USP CONTROLLED ROOM TEMPERATURE. This corresponds to information I obtained from the literature which says the 75 mg capsules can be stockpiled by civil authorities, no special conditions specified. Note that the conditions Gaudia cites are essentially ambient (room temperature) conditions.

Here is some additional information that may be of interest (source: Ward et al. "Oseltamivir (Tamiflu(R)) and its potential for use in the event of an influenza pandemic," Journal of Antimicrobial Chemotherapy (2005) 55, Suppl. S1l, i5 - i 21).

Side effects: This drug seems remarkably free of side effects, although, as with any drug, they do exist. Since its introduction in 1999, its manufacturer, Roche, estimates it has been given to about 20,000,000 people world-wide, although post-market surveillance is not well-developed. Roche claims that only one of every 10,000 patients has had one or more adverse reactions. During one review period, about 1000 of the 4000 adverse events were considered "serious." Most of the reports come from Japan. My interpretation: adverse reactions are very uncommon, but when they occur may be more serious than with other drugs.

Most adverse reactions were gastrointestinal (nausea, vomiting, diarrhea), dizziness and more rarely some allergic-type reactions (rash, hives, eczema, swelling of the face). On rare occasion the allergic reactions were serious (Stevens-Johnson Syndrome, erythema multiforme).

Use during pregnancy and while nursing: There is not enough information available to make a judgment of safety during pregnancy, so Roche's cautions that it be used only when potential benefits justifies potential risk to the fetus. They cite its use during pregnancy in 61 cases, among which there were 10 reports of abortion (of which 6 were therapeutic abortions, reason is not stated) and there was one each of trisomy 21 (Downs) and anencephaly. It is hard to know what to make of these data but I do not find them particularly reassuring, even though more than 80% of the pregnancies resulted in normal babies.

Based on animal studies the drug and its active metabolite would be expected to be excreted in milk during breast feeding. Therefore its use in lactating women is subject to the same strictures as for pregnancy: use only if the potential benefit for the lactating mother justifies the potential risk to the nursing infant.

Bottom line: this is a medicine, not a candy. Do not use without good reason. With that qualification, it is more than usually safe and should be used when indicated.

I have decided not to give dosages here at this time. This is a drug to be used on the advice and at the direction of a health care provider. In medicine we have an old saying, "If you treat yourself, you have a fool for a doctor and a fool for a patient."

Here are the image links from Gaudia (thanks!):