defects in your own unborn child. So, it’s quite understandable that the
mention of a possible birth defect with the use of finasteride (Propecia 1mg and
Proscar 5mg, Merck) would cause considerable anxiety and concern. The package
insert for Proscar states under the section on Contraindications: “Because
of the ability of Type II 5alpha reductase inhibitors to inhibit the conversion
of testosterone to DHT, finasteride may cause abnormalities of the external genitalia
of a male fetus of a pregnant woman who receives finasteride”. So, is the
risk real?
Theoretically, yes. Practically, no.
What Exactly Happens?
The specific birth defect they're referring to, which happens when there is no
Type II 5-alpha reductase enzyme in the male embryo, is called hypospadias. Hypospadias
is a birth defect in which the urinary tract opening is developed on the ventral
surface (under side) of the penis rather than at the tip of the penis. There have
never been congenital abnormalities observed in female fetuses at any
dosage of finasteride.
During the research and development phase of finasteride, studies were done
on experimental animals. Rats, rabbits, and rhesus monkeys were given finasteride
to determine its relationship to birth defects, i.e. hypospadias.
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Hypospadias did occur in the male offspring, when pregnant rats were administered
doses that were between 5 and 5000 times the amount recommended for men in treating
Male Pattern Baldness (1mg/daily). The critical period during which these effects
can be induced in male rats was determined to be during the 16th and 17th days
of gestation.
In rabbit fetuses which had finasteride directly injected into the uterus from
days 6-18 of gestation at doses equivalent to 5000 times the recommended human
dosage, no evidence of malformations was observed. This result would be expected,
since there was no exposure during the critical period of genital system development
in rabbits.
Conceiving a baby while on Propecia
 |
We have all seen the warnings about pregnant women being advised not to handle
a broken propecia tablet. In many people's minds, it stands to follow that if
they shouldn't handle a broken tablet, surely semen containing finasteride would
be dangerous as well. Again, we should always look to the studies to get the
facts we need. Scientists intravenously gave 750 times that dosage to Rhesus
Monkeys. In fact it was 750 times the highest possible dose a woman could be
exposed to through semen from a man taking 1mg per day. Even at this dose, there
were no genital abnormalities observed.
Propecia and Genital Development during Pregnancy
In the human embryo, the period of external genitalia development is during
the 7th - 9th weeks of gestation. Although the sperm (which can contain finasteride
from the man taking Propecia) plays a role in determining the sex of the baby
(either Y-bearing or X-bearing), the actual male and female morphological characteristics
do not begin to develop until the seventh week of Pregnancy.
Prior to this time, the genital systems of the two sexes are similar, and the
initial period of genital development is referred to as the ‘indifferent
state of sexual development’. About six weeks after conception, if a Y
chromosome is present in the embryo's cells (as it is in normal males), a gene
on the chromosome directs the undifferentiated gonads to become testes. If the
Y chromosome is not present (as in normal females), the undifferentiated gonads
will become ovaries. If the gonads become testes, they begin to produce androgens,
primarily testosterone, by about eight weeks after conception. These androgens
stimulate development of the one set of the genital ducts into the epididymes,
vas deferens, and ejaculatory duct. The presence of androgens also stimulates
development of the penis and the scrotum. Hypospadias can result if there is
inadequate production of androgens by the fetal testes.
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Propecia is already Long Gone
Since the sensitive period of development of the
external genitalia in the human embryo is the 7th to 9th weeks of gestation,
there can be no danger to the child if the father is taking finasteride at the
time of conception. Originally, Merck decided to err on the side
of caution and warned against the possible problem of finasteride transfer in
semen. This warning has since been removed from the package insert. Considering
the medical/legal implications of a theoretically possible link of finasteride
treatment to birth defects, it is reasonable to assume that Merck & Co.
must be very confident in knowing that impregnating a woman while taking finasteride
absolutely does not cause birth defects.
Intercourse During Pregnancy
Nor is there any evidence of birth defects when the father taking finasteride
has intercourse with the pregnant mother during the critical periods of sexual
development. The in utero effects of finasteride exposure during the period
of embryonic and fetal development (gestation days 20-100) were evaluated in
the rhesus monkey, a species fairly predictive of human development. Intravenous
administration of finasteride to pregnant monkeys at doses as high as 800ng/day
(at least 60 to 120 times the highest estimated exposure of pregnant women to
finasteride from semen of men taking 5mg/day) caused no abnormalities in male
fetuses.
Still, Merck retains this admonition: “Women should not handle crushed
or broken Propecia tablets when they are pregnant or may be potentially pregnant
because of the possibility of absorption of finasteride and the subsequent potential
risk to a male fetus. Propecia tablets are coated and will prevent contact with
the active ingredient during normal handling, provided that the tablets have
not been broken or crushed.”
In Conclusion
Considering that intravenous administration of finasteride to pregnant experimental
animals during the critical periods of sexual development didn’t cause
birth defects, there is no reason to believe that transdermal absorption of
finasteride from handling broken tablets could cause birth defects in the male
child. But, since Propecia has not been approved by the FDA for use by women,
Merck has nothing to lose by retaining this warning. In fact, it has good p.r.
value.
So, can finasteride cause birth defects? Yes, there is a theoretical possibility
that it can, but the probability is close to nil, when finasteride is taken
in the recommended dosages. Since Propecia was approved by the FDA on 22 December
1997 and Proscar on 28 August 1996, millions of doses of finasteride have been
taken and there has not been a single case report of a birth defect. Now that’s
reassuring information.
Richard Lee, M.D.
Andersson, S.; Berman, D. M.; Jenkins, E. P.; Russell,
D. W.: Deletion of steroid 5-alpha-reductase 2 gene in male pseudohermaphroditism.
Nature 354: 159-161, 1991
Greene, S. A.; Symes, E.; Brook, C. G. D.: 5-Alpha-reductase deficiency causing
male pseudohermaphroditism. Arch. Dis. Child. 53: 751-753, 1978
Imperato-McGinley, J.; Gautier, T.: Inherited 5-alpha-reductase deficiency in
man. Trends Genet. 2: 130-133, 1986
Moore, KL and Persaud, TVN: Before We Are Born-Essentials of Embryology and
Birth Defects; 6th Edition, Saunders 2003
New England J of Med: 1994, Jan 13; 330 (2) 120-5
Physicians Desk Reference, 58th Edition, 2004, pp. 2172-2178
Thigpen, A. E.; Davis, D. L.; Milatovich, A.; Mendonca, B. B.; Imperato-McGinley,
J.; Griffin, J. E.; Francke, U.; Wilson, J. D.; Russell, D. W.: The molecular
genetics of steroid 5-alpha-reductase 2 deficiency. J. Clin. Invest. 90: 799-809,
1992
http://encarta.msn.com/encyclopedia_761580700/Human_Sexuality.html#endads
http://science.howstuffworks.com/human-reproduction5.htm
Sex-organ development is determined by the third month of development.
http://www.people.virginia.edu/~rjh9u/sexdev.html
http://www.lindasy.com/cgi-local/SoftCart.100.exe/skin411/411_dec01.html?E+scstore
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