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Combivir Potency and temperature extremes.
Aug 2, 2001

Hello, I would like to say I am extremely grateful for your column. I am currently taking both sustiva, once a day in the morning and combivir, twice a day. I take the combivir at 13 hour intervals as closely as possible (6:00 AM & 7:00 PM). My morning adherance is not a problem as I take the combivir with the sustiva prior to leaving for work however, from time to time I am not able to get home in time (13 hour interval from the morning dosage) to take my second combivir. In order to maintain my adherance as closely a possible to the 13 hour interval, I carry a supply of combivir in my vehicle. My question is this, does the extreme heat build up in my vehicle during the day have any effect on the potency of the combivir? Similarly, during the winter months, would below freezing temperatures have any effect on the potency of the combivir? If I may, I have another question, a pharmacist I have spoken to in the past at the HIV clinic I attend has indicated that combivir (in particular) has a faily good 1/2 life and that the very occassional missed dosage is really of no great concern. Is he correct and what is the the time variance for both sustiva and combivir should on occassion it not be possible to take either at strickly exact times? The time variance I am speaking of would be a couple of hours give or take, either way. On occassion I have had need to travel with my job and I do encounter time changes. Thank you, I look forward to your reply.

Positive in Canada.

Response from Dr. Little

Regarding your first question - I am not absolutely sure, but we generally advise avoiding extremes of temperature to optimize the stability of the drug. I suspect that baking hot temperature of greater than 100 degrees F should be avoided, but I cannot say with any certainty that extended periods at this temperature will reduce their potency. What I do recommend is carrying your extra supply on your person, or perhaps at another safe/private location which would avoid the heat/freezing question. Freezing temperatures are not generally the problem, it is the heat.

Sustiva has a particularly long half life, longer than combivir. I agree that missing an occasional dose of either is not a cause for crisis - however, the problem is really when the occasional missed dose turns into 2-3 a week or so, then you are much more likely to have a problem. For someone who takes all of their pills regualary and almost never misses a dose - execpt on rare occasions - this is almost never a problem. In general, the shorter the half life of the drug, the closer we recommend you stick to the prescribed dosing interval. For combivir - the goal is every 12 hours - plus or minus an hour should not make a great deal of difference. As always after this, there is a relative risk which increases with the time deviation from the prescribed range - the greater the duration of time from the prescribed dose, the greater the risk - there is not a black and white deliniation after which the drug does no good. This is why we spend so much time trying to help people find a way to work these medications into their regular routine and then perhaps special tricks that may help keep you on schedule when variation from the routine are necessary.


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