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Studies conducted at the National Center for Toxicological Research (NCTR)
have shown that SMZ at high doses causes thyroid tumors in rats and mice.
Data showing tumor formation in laboratory animals always causes CVM to
apply the 1 in 1,000,000 risk assessment techniques rather than the 100-fold
or 1000-fold safety factor normally used (i.e. an ultra-conservative approach to
food safety).

The Center for Veterinary Medicine is moving carefully and deliberately
toward a position on SMZ. There are a number of scientific issues yet to be
considered. If the drug must be removed from use in certain animal species,
a notice of opportunity for hearing will be published providing interested
persons an opportunity to present their views and justify a hearing.

B. Beta Lactams

Beta lactams are the most commonly used drugs for treating mastitis.
Penicillin continues to be among the most important antibiotics available
because of their remarkable antibacterial activity, relative safety and low cost.
Their spectra are primarily gram-positive bacteria. The penicillin’s share many
characteristics with the cephalosporins group and these two classes are often
referred to as beta-lactam drugs.

 Penicillin G (Procaine, Potassium, and Benzathine)
 Cloxacillin
 Ampicillin
 Hetacillin
 Amoxicillin
 Ticarcillin

C. Cephalosporin

Cephalosporin antibiotics are the most frequently prescribed class of
antibiotics. They are related to the penicillin’s and are a member of the beta
lactam antibiotics. The name is derived from the fact they were first isolated
from cultures of Cephalosporium acremonium from a sewer in Sardina. They
are used to treat respiratory tract infections, skin infections, urinary tract
infections and as a surgical prophylaxis before, during and after surgery they
are classified by "generations" 1-4 with the earlier numbered classes
predominantly effective on gram positive bacteria and the later classes or

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