Garlic (Allium sativum)
Garlic is used for its potential to prevent atherosclerosis by
reducing blood pressure, thrombus formation, and serum lipid and cholesterol levels.
It is also used in the treatment and prevention of cancer and infectious diseases.
The most compelling evidence of its therapeutic efficacy is in lowering serum cholesterol,
but this effect appears to be modest.[32]
The development
of concentrated garlic preparations has made it possible for patients to take otherwise
unachievable high doses, which may increase the risk of adverse effects.[33]
The pharmacologic effects of garlic are primarily attributed to
organosulfur compounds, particularly allicin and its transformation products. Several
of these compounds inhibit platelet aggregation in a dose-dependent manner.[34]
[35]
[36]
[37]
The inhibition of platelet aggregation by one of these compounds, ajoene, appears
to be irreversible and may potentiate the effect of other compounds such as prostacyclin,
forskolin, indomethacin, and dipyridamole.[38]
The mechanism by which these effects occur is
unknown, although some investigators have implicated the cyclooxygenase pathway.
[39]
Others have found a direct interaction with
the platelet fibrinogen receptor.[40]
The exogenous
adenosine in garlic[41]
and inhibition of endogenous
adenosine deamination and cyclic AMP phosphodiesterase may also play a role.[39]
The clinical significance of garlic's antiplatelet activity is
uncertain. In volunteers, inhibition of platelet aggregation to 5-hydroxytryptamine
was transient but potent[42]
; another study showed
no such activity.[43]
Inconsistent experimental
findings may be explained by differences in garlic preparations. Garlic may also
increase the risk of bleeding by promoting fibrinolysis, a finding that has been
demonstrated in volunteers.[44]
There is one case
in the literature of an octogenarian who developed a spontaneous epidural hematoma
that was attributed to heavy use of garlic.[45]
Although data are unreliable, bleeding times were significantly elevated when the
patient was hospitalized and returned to normal 3 days after the discontinuation
of garlic.
Although there are insufficient pharmacokinetic data for the constituents
of garlic, the potential for irreversible inhibition of platelet function warrants
the discontinuation of garlic at least 7 days before surgery, especially if intraoperative
or postoperative bleeding is a special concern or other platelet inhibitors are used.
