The presence of drug or antibiotic residues in milk and meat is illegal. Milk supplies containing detectable concentrations are not acceptable. Unless drug residues are avoided to protect milk’s reputation as a healthy, safe food, the market becomes jeopardized. Consumers want to be confident that their food supply is free of contamination by herbicides, pesticides, drugs, or antibiotics. Approximately 5-10 percent of the population is hypersensitive to penicillin or other antibiotics and suffers allergic reactions (skin rashes, hives, asthma, anaphylactic shock) at concentrations as low as 1 ppb penicillin. There is concern that small amounts of certain antimicrobial agents may significantly shift the resistance patterns in the microbial population in the human intestinal tract.

Antibiotics are known to interfere with the manufacture of several dairy products. Concentrations of 1 ppb delay starter activity for cheese, butter, and yogurt. Antibiotics also decrease the acid and flavor production associated with butter manufacture, and they reduce the curdling of milk and cause improper ripening of cheeses.


Drugs are administered to dairy cows for treatment of mastitis through intramammary or intravenous infusions and for disease therapy by intramuscular or intravenous injections, oral administration, feed supplementation, or reproductive infusions. FDA surveys indicate that improper use of drugs in the control of mastitis is the major source of residues found in the milk supply. The beef industry claims that a great percentage of the drug residues found in beef carcasses are in those of culled dairy cows.

Many drugs are retained in the animal body for longer times than indicated by label discard times. Consequently, milk samples remain positive for residues. A good example is penicillin with a recommended milk discard time of 72 hours. However, penicillin residue has persisted in milk for as long as 18 days. Other drugs may cause residues. At the end of the label withholding time, we have detected drug residues in 35% of cows treated with cephapirin for mastitis and 27% of cows given intramuscular injections with penicillin (Table 1). Some of the cephapirin treated cows were still positive at 48 hours after the recommended milk discard time. Drugs administered for dry cow therapy do not appear to cause drug residues if milk is not shipped for the first four days after calving, if dry periods are l’s recommendations are followed, dry cow therapy should not result in residues after calving. However, residues are possible and fresh cows should be tested, especially cows with short dry periods.

Suspected reasons for drug residues include: 

  • Extended usage or excessive dosage of approved drugs.
  • Poor records of treatment.
  • Milker or producer mistakes—accidental transfer into bulk tank.
  • Failure to observe recommended label withdrawal time.
  • Lack of advice on withdrawal period.
  • Prolonged drug clearance.
  • Treated-animal identification problems.
  • Multiple dosing.
  • Products not used according to label directions.
  • Withholding milk from treated quarters only.
  • Contaminated milking equipment.
  • Early calving or short dry periods.
  • Purchase of treated cows.
  • Use of dry cow therapy to lactating cows

Antibiotic Residues – A Global Health Hazard

Pathological Effects produced by Antibiotic Residues in Food

  • Transfer of antibiotic resistant bacteria to the human.
  • Immunopathological effects
  • Autoimmunity
  • Carcinogenicity (Sulphamethazine, Oxytetracycline, Furazolidone)
  • Mutagenicity – Nephropathy (Gentamicin)
  • Hepatotoxicity
  • Reproductive disorders
  • Bone marrow toxicity (Chloramphenicol)
  • Allergy (Penicillin)

Techniques used for Detection and Analysis of Drug Residues

  • HPLC
  • Liquid chromatography
  • Gas chromatography
  • Paper chromatography