I took 10mg amitriptyline for sleep for over a year and never got any side effects from it, nor did it lose effectiveness over that time, but that's just me. My Mum has been taking it for years at low dose for neuropathic pain and it has lost its effectiveness and she's needed other meds for her fibro. I couldn't find any side effects that were specified to be from long term use in the BNF (British National Formulary - UK prescribing guide), so I guess it could be any of the normal listed side effects, but less risk due to low dose maybe(?).

*Willow*