Hailed as a wonder drug since the mid-1980s, fluoxetine – the drug in Prozac – is on the World Health Organisation List of Essential Medicines. Used to treat human psychiatric disorders such as depression and post-traumatic stress disorder, fluoxetine is believed to increase levels of the neurotransmitter serotonin in the synapse between nerves by reducing reabsorption into nerve cells to relieve symptoms. But humans may not be the only animals on the doctor's prescription. Teresa Dzieweczynski and colleagues from the University of New England, USA, explain that the drug is turning up in our river systems, thanks to its resistance to breakdown, and seems to be affecting the behaviour of aquatic residents, reducing aggression and risk-taking, and affecting how often animals feed. But little is known about the longer-term effects of exposure or how the drug may alter behaviour in different settings. As female Siamese fighting fish are known to be less aggressive and inquisitive after a dose of the drug, Dzieweczynski and her colleagues Brennah Campbell and Jessica Kane decided to find out how male Siamese fighting fish that had not been exposed to the fluoxetine and those that had experienced a week-long low (0.5 μg l−1 – similar to the levels found in effluent water) or high dose (5 μg l−1 – similar to levels found in body fluids of human patients) of fluoxetine responded to situations that were designed to challenge their courage, and how their reactions changed over time.

Right from the start, it was clear that fish that had not been exposed to fluoxetine were much keener to explore a novel empty tank than the fish that had been swimming in water laced with the anti-depressant, and the fish that had received the highest dose were the most timid. However, the fish that had been exposed to 0.5 μg l−1 of the drug were bolder on some occasions than others. And when the team tested the fish's reactions to an unfamiliar tank furnished with intriguing stones and plants, or the presence of a shoal of lady Siamese fighting fish, the two sets of drugged males were equally disinterested in exploring; the strength of the drug did not affect them differently. The team says, ‘Perhaps most importantly and alarmingly, the effects of exposure lasted even after fluoxetine was removed’, and they warn, ‘Even brief periods of exposure could potentially produce chronic effects, especially as boldness is important in migration, aggression and predator evasion’.

T. L.
B. A.
J. L.
Dose-dependent fluoxetine effects on boldness in male Siamese fighting fish
J. Exp. Biol.