Yes, one swallow does not make a summer. There have been cases with unpleasant side effects which I remember when a vet nurse, but cannot provide written evidence.
It used to be that progesterone was used for chemical castration, and that’s no longer the case. There were many potential common side-effects with this therapy.
With Suprelorin, the risks are very few, and very, very unlikely (less than one in 10,000):
4.6 Adverse reactions (frequency and seriousness)
Moderate swelling at the implant site was commonly observed for 14 days during safety/efficacy studies.
During the treatment period, rare clinical effects have been reported: hair coat disorders (e.g. hair loss, alopecia, hair modification), urinary incontinence, down-regulation associated signs (e.g. decrease in testicle size, reduced activity). In very rare cases, a testicle may be able to ascend the inguinal ring.
In very rare cases, there has been transitory increased sexual interest, increased testicle size and testicular pain immediately after implantation. These signs resolved without treatment.
In very rare cases, a transient behavioural change has been reported with the development of aggression (see section 4.4).
The frequency of adverse reactions is defined using the following convention:
- very common (more than 1 in 10 animals treated displaying adverse reaction(s))
- common (more than 1 but less than 10 animals in 100 animals treated)
- uncommon (more than 1 but less than 10 animals in 1,000 animals treated)
- rare (more than 1 but less than 10 animals in 10,000 animals treated)
- very rare (less than 1 animal in 10,000 animals treated, including isolated reports).
https://www.ema.europa.eu/en/docume...on/suprelorin-epar-product-information_en.pdf