Analgesics are commonly employed drugs for perioperative procedures and are required for painful procedures. They include not only non-steroidal anti-inflammatory drugs (NSAIDs) but also opioids and local anesthetics. Although NSAIDs may be adequate for minor procedures, where light pain would be expected, opioids should be administered for more painful procedures since they may provide the highest analgesic efficacy. Among them, potent full-agonist opioids should be considered for severe pain. Local anesthetics can be also provided to block pain when administered locally, for example at the wound site. To improve analgesic efficacy NSAIDs and opioids, but also local anesthetics, are usually combined, thus reducing their doses a related side effects.

Unfortunately, analgesics are not devoid of side effects. NSAIDs block the inflammatory response and may not only interfere with research results when this response is evaluated, but also promote a reduction in renal and gastrointestinal blood flow, which may become dangerous when hypotension is favored by the use of inhalation anesthetics. Local anesthetics should be administered considering the maximum dose since their nerve blocking actions may not only affect nociceptive stimulation but also other vital organs such as the heart or the brain. Opioids have been found to produce not only the expected analgesic (hypoalgesic) action but also a delayed opposite action (hyperalgesia) which may appear in the postoperative period. Other analgesics may share this dual and opposite action.  It remains unclear whether this hyperalgesic effect has clinical relevance in the painful animal or should be considered only in the healthy non-painful animal suggesting indiscrimate use of opioids should be avoided.