Pregnant and nursing animals in pain need special consideration because clinical trials of analgesics usually exclude these populations and safety data are lacking.
Pregnant and nursing animals in pain need special consideration because clinical trials of analgesics usually exclude these populations and safety data are lacking. A few recent review articles and guidelines with helpful information are summarized here.
Pregnancy causes physiologic changes that can alter pharmacokinetics and pharmacodynamics, Karol Mathews, DVM, DVSc, noted in an article published in Veterinary Clinics of North America: Small Animal Practice.
For example, higher cutaneous blood flow can increase the absorption of topical medications. Dr. Mathews notes. Greater total body water increases the volume of drug distribution, and higher total body fat lowers the plasma availability of lipid-soluble drugs. Changes in liver enzyme activity affect the breakdown of drugs that undergo hepatic metabolism, and increased renal function escalates the elimination of water-soluble drugs and their metabolites.
Furthermore, drugs and their metabolites can cross the maternal-fetal barrier and can reach potentially hazardous concentrations in milk. Counterbalancing these risks are the need for humane care and the fact that both chronic and severe acute pain contribute to poor perinatal outcomes. Thus, the risks and benefits of analgesia in pregnant and nursing animals merit case-by-case consideration.
Opioids tend to be the analgesic of choice for pregnant dogs and cats. Teratogenicity and other adverse fetal effects are generally not a concern, except in the rare case that a patient needs long-term opioids, according to Dr. Mathews.
The World Small Animal Veterinarian Association (WSAVA) recommends hydromorphine and morphine over meperidine, butorphanol, or nalbuphine, which are more lipid-soluble, and therefore can reach higher concentrations in the fetus.
Perhaps the most common use of opioids in pregnant animals is before cesarean section. Most newborns do not show adverse effects with this use, but if they seem depressed, WSAVA recommends placing a drop of naloxone under the tongue. Dr. Mathews recommends repeating this dose in 30 minutes if the depression persists.
For lactating animals that need ongoing opioids, nursing should be timed for just before the next dose, and opioids with long half-lives should be avoided, according to Dr. Mathews.
Local anesthetics are generally considered safe and non-teratogenic, and can be a good alternative to systemic analgesia, according to WSAVA. For example, local infiltration of the cesarean incision line can reduce the need for systemic analgesia. Because lidocaine and its metabolites are not lipophilic, concerns about concentration in milk are minimal, WSAVA notes.
Nonsteroidal anti-inflammatories are potentially teratogenic and may adversely affect development. They should be avoided in pregnant animals, and should be used sparingly — if at all – in lactating animals, according to WSAVA. Dr. Mathews recommends no more than one dose of an NSAID after cesarean section.
Although ketamine is not known to be teratogenic, it increases uterine tone, and therefore should be avoided during pregnancy, according to WSAVA. Ketamine usually is not recommended for lactating animals because of the chances of its passing into milk.
Xylazine inhibits uterine perfusion and should be avoided during pregnancy, while it is unclear how medetomidine and dexmedetomidine affect pregnant dogs and cats, WSAVA notes. The organization makes no recommendations about these drugs for nursing animals.
WSAVA recommends avoiding herbal analgesics in pregnant and lactating animals because of a lack of safety data.
Carefully monitor pregnant and nursing dogs and cats for adverse effects of analgesia. When in doubt, halt treatment and consult with the prescribing veterinarian. Complex cases may benefit from consultation with experienced specialist, such as a veterinary anesthesiologist or theriogenologist (veterinary reproductive specialist).
If pain relief medication is given for several days, the dose should gradually be tapered instead of stopping abruptly, unless there is concern about a serious adverse effect.
As always, avoid giving over-the-counter analgesics intended for humans, particularly without input from an experienced veterinarian. Acetaminophen (paracetamol, Tylenol), ibuprofen, naproxen, and acetylsalicylic acid have caused many fatal toxicities in dogs and cats.
In terms of nonpharmacologic relief, common-sense measures such as bandaging, gentle heat, and cold packs can help relieve discomfort, but are best used in consultation with a veterinarian or experienced veterinary technician.
Dr. Amy Karon earned her doctorate in veterinary medicine and master’s degrees in public health and journalism from the University of Wisconsin-Madison. She was an infectious disease epidemiologist and “disease detective” (EIS officer) with the Centers for Disease Control and Prevention before becoming a full-time medical writer. She lives in the San Francisco Bay area, where she volunteers for the local Humane Society.