In rats, administration of nandrolone resulted in reduced dopaminergic and serotonergic transmission in the brain in response to the administration of cocaine for a period lasting from 5-6 times the duration of administration.
If you look at anecdotal reports on the Steroids subreddit, we see many men complaining of Erectile Dysfunction, Depression, Lactation (From elevated prolactin) for months to years after a cycle of nandrolone.
Nandrolone also directly agonizes the progesterone receptor which works synergistically with estradiol. This can potentiate gynecomastia, lethargy etc.
Regarding the negative effects of androgenic anabolic steroids (AASs) abuse, the long-term effect of nandrolone decanoate with/without two intensities of endurance exercise training was investigated on heart tissue and electrocardiogram (ECG) in ...
www.ncbi.nlm.nih.gov
This study above found that nandrolone increased left ventricular hypertrophy with and without exercise. Granted, the rats were given 5 mg/kg twice per week but you'd have to convert the dosage to human equivalent terms, you'd need to divide by 6.2 (
https://www.fda.gov/media/72309/download). That comes out to 0.806 mg/kg twice per week. For a 100 kg male, that would be 80.6 mg twice per week or 160 mg total. That is a dose that some men use as an adjunct to TRT.
If you use a low dose, you'll likely be fine. However, I personally don't see the rewards as being worth the risk. If you stay under 100 mg per week you will likely minimize side effects.
All this being said, there are no direct human trials on the safety of nandrolone. However, we have seen plenty of problems arise for AAS users.
Do what you want with this information, but I would personally advise you to steer clear of 19-Nors.