Promethazine and False Positive Pregnancy Tests

November 8, 2014 3:32 AM

Does anyone know how long Promethazine/Phenergan is thought to interfere with home pregnancy tests?
posted by pairofshades to Health & Fitness (3 answers total)

Well, the half-life of phenergan is 7-14 hours in the bloodstream assuming normal liver function (promethazine is hepatically cleared from the body).

That means you will clear 95% of it from your circulation (and from your urine) in 5 half lives, or not more than 14 * 5 = 70 hours, or about 3 days. You will clear 99% of it in 7 half lives, or 14 * 7 = 98 hours, or about 4 days.

I've looked around to see if there's any specific literature on this. Unfortunately, I think the above is the best answer you're likely to get.
posted by killdevil at 7:30 AM on November 8, 2014 [1 favorite]

I think it would be very hard to predict, so if I had used Phenergan recently and got a positive test, I would ask my doctor about getting a blood test (if negative test, I'd probably just wait and test again later, although I know that's easy for someone who's not holding their breath for the test results to say). I am not sure what the mechanism of action is by which the drug interferes with the pregnancy test - if it has hormonal effects that increase the blood hCG level, then it may not be as simple as waiting for the drug to be cleared per killdevil's response.
posted by treehorn+bunny at 11:47 AM on November 8, 2014 [4 favorites]

Response by poster: Thanks for responding everyone. I've flagged the post to the mods because I wrote "false positive"- I think that its more likely that antihistimines cause false negatives.. but in any case it wouldn't be good if anyone was inspired to take promethazine to mess with their pregnancy test.
posted by pairofshades at 10:05 PM on November 8, 2014