(1) Drug factors ① Effects of dosage, dosage form and route of administration: When the dosage is too small, the curative effect is poor; If the dose is too large, it will lead to toxic reaction or lead to animal poisoning and death. The absorption of aqueous solution and injection is faster than that of oil, suspension and solid preparation. The order of effect of different routes of administration is intravenous administration > inhalation administration > intramuscular injection administration > subcutaneous injection administration > oral administration > rectal administration > transdermal administration. ② Time and frequency of administration A. Administration on an empty stomach before feeding: fast and sufficient absorption, fast and good efficacy.
B. Administration after feeding: to avoid direct damage to gastrointestinal mucosa, it is suitable for drugs with strong irritation.
C. Dosage: It depends on the condition and the half-life of the drug. ③ Repeated medication A. Tolerance: After continuous medication, the curative effect will gradually decrease, and it is necessary to increase the dosage to achieve the original effect.
B drug resistance: it means that pathogens, parasites and tumor cells are less sensitive to drugs, and the curative effect of drugs is reduced or ineffective. ④ Drug Interaction Tetracycline, enrofloxacin, etc. can be complexed with metal ions such as calcium, iron and magnesium in the digestive tract, affecting drug absorption or inactivating drugs.
(2) Animal factors ① Physiological factors The response of pigs to the same drug varies with age, sex, pregnancy or lactation, which is closely related to the functional state of organs and tissues, especially to the drug metabolism enzyme system in the liver. For example, the liver detoxification function and kidney detoxification function of newborn piglets are weak. Therefore, the half-life of drugs eliminated by liver microsomal enzyme metabolism and renal excretion in young piglets will be prolonged. Older pigs also have the above phenomenon (decreased liver and kidney function), and are generally more sensitive to drugs than adult animals, so the clinical dosage should be reduced appropriately. ② The functional state of the body is different (pathological state)
Many drugs play a significant role in sick animals, even in pathological state. For example, antipyretic and analgesic drugs can cool sick pigs, but have no effect on normal pig body temperature. Digitalis has a good cardiotonic effect on chronic congestive heart failure, but has no obvious effect on the normal heart.
Severe liver and kidney dysfunction in pigs can affect the biotransformation and excretion of drugs, cause the accumulation of drugs, prolong the half-life, and thus enhance the role of drugs, which may lead to toxic reactions in severe cases. However, there are also a few drugs that have no effect until the liver biotransformation, such as cortisone and prednisone, and their effects are weakened in sick pigs with liver dysfunction.
In pigs with severe parasitic diseases, hemorrhagic diseases or malnutrition, the plasma protein is greatly reduced, which can increase the blood free drug concentration of drugs with high plasma protein binding rate, which can not only enhance the drug action, but also increase the biotransformation and excretion of drugs and shorten the half-life. (3) Individual difference means that the blood concentration, action and action maintenance time of the same dose of drugs are different in different individuals. Can be divided into high sensitivity and tolerance. Under the same basic conditions, a few individuals of the same animal are particularly sensitive to drugs, which is called high sensitivity; A few individuals are particularly insensitive, which is called tolerance. Individual differences not only show the difference of drug dosage, but also show qualitative differences, that is, individual animals have allergic reactions after taking drugs.
(3) Environmental factors, including poor feeding management, moldy feed, changes in external environment and seasonal changes, can all affect the drug action.