This updated guideline on progesterone-only implants is from the FSRH. There a few points of interest. Some just clarify old advice, others are completely new. I've listed the most useful bits:
Replacing the implant. If the implant is replaced within 3 yrs of insertion, there is no need for extra precautions as it is rare for ovulation to occur within 7d of its removal.
Dysmenorrhoea. The implant may improve dysmenorrhoea. In endometriosis, there are some small studies that suggest it may be helpful, but its use is not included in current guidelines.
Risks. There may be a small increased risk of breast cancer, though this risk goes on stopping the implant. There is little or no increased risk of VTE, MI, CVA or lower bone mineral density.
Acne. Acne may worsen, improve or start with the implant.
Side-effects. Some women relate headaches or changes in mood, libido or weight with the implant, but there is no current evidence of a causal association.
Bleeding Patterns. The bleeding pattern in the first 3m of use is broadly predictive of future bleeding patterns. 33% report infrequent bleeding (< 2 episodes of bleeding / spotting in 90d). 21% report amenorrhoea. 17% report prolonged bleeding (1 or more episodes of bleeding / spotting lasting 10 or more days in 90d) and 6% report frequent bleeding (> 4 episodes of bleeding or spotting in 90d).
How to manage bleeding problems. STD screen. Check they are up to date with cervical smears if appropriate. The COC can be used cyclically or continuously for 3m. There is no evidence looking at longer term use, so use after this should be based on 'clinical judgment'. Similarly use of the POP is based on clinical judgement. If the bleeding is towards the end of the 3 yrs, consider an early implant change.
Weight and use of the implant. It is safe to use the implant at any weight. Studies haven't shown an increased risk of pregnancy with increased weight, but a lower duration of effect can't be ruled out. The FSRH suggests that for women over 70kg, they should be warned that efficacy may be lower as time goes by. They don't currently advise an earlier replacement, but if the woman requests this, it should be done. There isn't any evidence as to when this earlier replacement should be. The problem is that the hormone levels may drop low enough to allow ovulation, but it may be that the other contraceptive effects are maintained (eg the thickened cervical mucous).