Can anyone give a recent reference for the potential interaction between
TCN and Birth control pills. This antibiotic seems to be the only
medicine stopping my patient's acne after all other orals and topicals
and accutane.

Martin H. Kay Ph.D.,M.D.
------------------------ 
 
 
Medications associated with a possible risk of contraceptive failure:
With the IUD:

azathioprine  (Zerner J, Doil KL,  Drewry J, Leeber DA,  Intrauterine
contraceptive device failures in renal transplant patients. J Reprod Med 1981
Feb;26(2):99-102.) 

nonsteroidal anti-inflammatory agents (Papiernik E,Rozenbaum H, Amblard P,
Dephot N, de Mouzon J, Intra-uterine device failure: relation with drug use.
Eur J Obstet Gynecol Reprod Biol 1989 Sep;32(3):205-12)   

With the oral contraceptive failure has been questionable associated hepatic
enzyme inducers (griseofulvin and rifampin) may increase metabolism of
estrogen due to induction of hepatic microsomal enzymes.  (USP-DI. Drug
information for the health care professional, Vol 1.  Taunton, Mass: Rand
McNalley, 1995, pp. 1258,66.)  

penicillin and ampicillin  may stimulate estrogen metabolism or reduce
enterohepatic circulation of estrogens.(USPDI)  

tetracyclines, sulfonamides and tranquilizers may increase breakthrough
bleeding and reduce contraceptive effectiveness.  (USPDI.)  

itraconazole -case report of pregnancy occurring with use. (Pillans PI,
Sparrow MJ.  Pregnancy associated with a combined oral contraceptive and
itraconazole [letter}.  New Z Med J 1993:106:436.) 

Barbara R. Reed, MD
-------------------


Be very careful about accepting at least some of these references at face 
value. All contraceptives fail even under ideal circumstances, lots of 
people take drugs, and it's awfully easy to make an association which may 
be spurious seem believable. It's been a while since I actually read the 
source literature, but at least for TCN and ampicillin in 1991, it's 
incredibly weak evidence.

Mark Ling, M.D., Ph.D.
----------------------


There is an interesting letter from doctors Morgan & Carmichael to the editor
of the Journal of Dermatologic Treatment, Vol. 6(4):250; Dec.1995. They
review the theoretical mechanism of antibiotic-induced OC failure: decreased
enterohepatic recirculation of estrogen due to inhibition of gut flora, e. g.
by Tetracycline.  They point out that this effect is only likely to occur
during the first 2 weeks of tetracycline therapy, since the gut flora rapidly
acquire resistance to it.   They further report that statistics gathered by
the UK Committee on Safety of Medicines since 1987 lead to the conclusion
that the incidence of pregnancies in women on combined antibiotic/OC therapy
is no greater than the intrinsic failure rate of OC's alone.  Their
suggestion is to instruct patients to use a barrier method of contraception
during their first two weeks on tetracycline, and then carry on as usual.

Gene Sienkiewicz, M. D.
-----------------------


I am in agreement with Mark Ling about the weak association between oral
contraceptive (OC) failure and drug usage.  OCs have an intrinsic failure
rate, which is argued but usually estimated to be less than one per cent.
  That is under ideal conditions.  

When faiure occurs and the woman was taking another drug, especially one
which has had an association, however weak, with OC failure, it is the other
drug and not the intrinsic failure rate of the OC which will be blamed.  That
is human nature, not science.  Science is tough to come by under these
circumstances.

It was in this spirit that these references were offered--this spirit and the
one that wants to know what an attorney might find that I wish I had known
earlier.

Barbara R. Reed, MD
--------------------


Best one I think is Am J Ob & Gyn 164;1991: 28-33 where women were given
tetracycline 500 mg qid (!) and there was NO change in plasma ethinyl
estradiol or norethindrone levels.

This is the only article I know of where there was scientific experimentation
(instead of idle speculation, anecdotes and medico-legal mumbo-jumbo).

The fact remains that certain other antibiotics (notably rifampin, and
perhaps also ampicillin and TMP/SMX) which can interfere with the
effectiveness of BCP.

Kevin C. Smith MD FRCPC
-----------------------