Ouabain /wɑːˈbɑːɪn/ or /ˈwɑːbeɪn, ˈwæ-/ (from Somali waabaayo, "arrow poison" through French ouabaïo) also known as g-strophanthin, is a plant derived toxic substance that was traditionally used as an arrow poison in eastern Africa for both hunting and warfare. Ouabain is a cardiac glycoside and, in lower doses, can be used medically to treat hypotension and some arrhythmias. It acts by inhibiting the Na/K-ATPase, also known as the sodium–potassium ion pump. However, adaptations to the alpha-subunit of the Na⁺/K⁺-ATPase via amino acid substitutions, have been observed in certain species, namely some herbivore insect species, that have resulted in toxin resistance.

It is classified as an extremely hazardous substance in the United States as defined in Section 302 of the U.S. Emergency Planning and Community Right-to-Know Act (42 U.S.C. 11002), and is subject to strict reporting requirements by facilities which produce, store, or use it in significant quantities.

Ouabain can be found in the roots, stems, leaves, and seeds of the Acokanthera schimperi and Strophanthus gratus plants, both of which are native to eastern Africa.

Ouabain is a cardiac glycoside that acts by non-selectively inhibiting the Na⁺/K⁺-ATPase sodium–potassium ion pump. Once ouabain binds to this enzyme, the enzyme ceases to function, leading to an increase of intracellular sodium. This increase in intracellular sodium reduces the activity of the sodium–calcium exchanger (NCX), which pumps one calcium ion out of the cell and three sodium ions into the cell down their concentration gradient. Therefore, the decrease in the concentration gradient of sodium into the cell which occurs when the Na/K-ATPase is inhibited reduces the ability of the NCX to function. This in turn elevates intracellular calcium. This results in higher cardiac contractility and an increase in cardiac vagal tone. The change in ionic gradients caused by ouabain can also affect the membrane voltage of the cell and result in cardiac arrhythmias.

An overdose of ouabain can be detected by the presence of the following symptoms: rapid twitching of the neck and chest musculature, respiratory distress, increased and irregular heartbeat, rise in blood pressure, convulsions, wheezing, clicking, and gasping rattling. Death is caused by cardiac arrest.

Ouabain is a highly toxic compound, however, it has a low bioavailability and is absorbed poorly from the alimentary tract as so much of the oral dose is destroyed. Intravenous administration results in greater available concentrations. After intravenous administration, the onset of action occurs within 2–10 minutes in humans with the maximum effect enduring for 1.5 hours.

Ouabain is eliminated by renal excretion, largely unchanged.

In 1991, a specific high affinity sodium pump inhibitor indistinguishable from ouabain was first discovered in the human circulation and proposed as one of the potential mediators of long term blood pressure and the enhanced salt excretion following salt and volume loading. This agent was an inhibitor of the sodium pump that acted similarly to digitalis. A number of analytical techniques led to the conclusion that this circulating molecule was ouabain and that humans were producing it as an endogenous hormone. A large portion of the scientific community agreed that this inhibitor was endogenous ouabain and that there was strong evidence to indicate that it was synthesized in the adrenal gland. One early speculative interpretation of the analytical data led to the proposal that endogenous ouabain may have been the 11 epimer, i.e., an isomer of plant ouabain. However, this possibility was excluded by various methods including the synthesis of the 11 epimer and the demonstration that it has different chromatographic behavior from ouabain. Critically, the primary observations concerning the identification of ouabain in mammals were repeated and confirmed using a variety of tissue sources on three different continents with advanced analytical methods as summarized elsewhere.