Apocynum, more commonly referred to as Dogbane, is a genus that comprises approximately 70 species of plants. In North America, Apocynum is commonly represented by two primary species, Apocynum androsaemifolium, or Spreading Dogbane, and Apocynum cannabinum, commonly called Dogbane or Indian Hemp, along with one hybrid, Apocynum × floribundum, often referred to as Western Dogbane. Although at one time as many as seven North American species were recognized due to the complex variations the genus exhibits, these three forms are commonly discussed together because of their similar growth habits, appearance, and toxic principles.

Both A. cannabinum and A. androsaemifolium are distributed throughout much of North America, whereas A. × floribundum is primarily limited to the western half of the continent, with smaller populations occurring along the Mason-Dixon line in the East. A. androsaemifolium is typically found along the edges of woods and in dry thickets, whereas A. cannabinum prefers a moister environment and can commonly be found in damp ditches, along the edges of waterways, and in open woodlands and fields. Apocynum × floribundum is more versatile and may be found in a variety of dry or moist soil conditions, especially in rocky areas.

Apocynum spreads prolifically and can often be found in large colonies due to a long horizontal rootstock that develops from an initial taproot. All three North American varieties are perennial flowering herbs and contain cardiac glycosides or cardenolides, including cymarin, cyntoxin, also known as apocynamarin, and strophanthidin. ASPCA lists Apocynum androsaemifolium, under Indian Hemp, as toxic to dogs, cats, and horses, with cardenolides identified as the toxic principle and clinical signs including diarrhea, weakness, and slow heart rate.

The leaves are smooth, not notched or toothed, and arranged oppositely along the stem. The lower leaves will typically have short stems, while the upper leaves may be nearly stemless. In the fall, the leaves turn yellow and then drop to the ground. The flowers are small, approximately 1/4 inch wide, bell-shaped, and may be white, pinkish, or greenish-white. They are produced in terminal clusters. The plant blooms in late spring, and the flowers may remain until late summer.

All parts of the plant, whether fresh or dried, should be considered toxic. This point is especially important because dogbane may become a hay contaminant. USDA notes that dogbanes contain cardiac glycosides with physiologic actions similar to digitoxin, that the principal toxin has been identified as cymarin, and that both dried and green plant material are toxic. Dogbane is sometimes mistaken for milkweed, another group of toxic plants that may also contain cardiac glycosides. The two differ in part because dogbane has fine hairs on the underside of the leaves and tends to have a bitter, sticky, milky-white sap.

In spite of the fact that these species are relatively common and often very abundant, they are rarely responsible for either companion-animal or livestock deaths. There is little doubt that the plant is toxic; however, like the tomato, the actual danger it poses to pets may be somewhat exaggerated in ordinary household exposure scenarios. First, the plant is extremely fibrous, making it exceptionally difficult to eat. Second, the taste is so bitter as to make it barely palatable. As a result, livestock will generally ignore the plant when better forage is available, and companion animals will rarely ingest enough to suffer serious intoxication.

Setting aside the general lack of palatability, the actual amount of toxin in the plant may vary substantially based upon growing conditions, plant part, age of the plant, and whether the exposure involves fresh plant material or dried contamination in hay. In a study conducted on cattle and sheep, the animals were given Apocynum leaves at amounts equal to 1% and 5.6% of their body weight, respectively. In neither case did the animals show adverse effects. However, horses have reportedly died from eating hay that contained only a small amount of Apocynum leaves and stems. In another study, a 105 lb sheep was given more than 5 lbs of leaves and suffered only diarrhea.

In the vast majority of cases, the earliest and most common sign of Apocynum ingestion is diarrhea, with or without blood. Other, less common but nevertheless important effects include symptoms of cardiac insufficiency such as weakness, cold extremities, slow heart rate, bradycardia, and arrhythmias. There may also be mydriasis, sweating, and a compensatory increase in heart rate in some animals. Because of the very fibrous nature of the plant, impaction or constipation may also occur, particularly when larger amounts of coarse plant material are consumed.

The main practical concern with Bitter Root and other dogbanes is not that every nibble will be fatal, but that the plant contains true cardiac-active compounds. Cardiac glycoside and cardenolide poisoning can become serious when enough plant material is consumed, when dried plant material contaminates hay, when a vulnerable animal is exposed, or when cardiac signs develop. Any animal showing weakness, cold extremities, abnormal heart rhythm, collapse, or slow heart rate after possible dogbane ingestion should be treated as a veterinary priority.