The main use of St John's Wort is for depression but there are supposedly other uses (WebMD, n.d.) which may or may not work.
The only study I can find on the effectiveness of St John's Wort against mild to moderate depression is Brattström (2009) which you linked to in the comments. The study looked at safety (adverse event frequency) and influence on depression (HAM-D, CGI). One key point I would like to make is that the report states that symptoms of depression did subside;
Mean HAM-D scores decreased steadily from 20.58 at baseline to 12.07 at week 26 and to 11.18 at week 52. Mean CGI scores decreased from 3.99 to 2.20 at week 26 and 2.19 at week 52
Two hundred and seventeen (49%) patients reported 504 adverse events.
General Safety of St John's Wort
The adverse events reported by Brattström (2009) could have been for a number of reasons and Brattström only attributed 30 (6%) to the treatment.
There are many drugs which St John's Wort interacts badly with including antidepressants and birth control pills (WebMD, n.d.; British National Formulary, 2019).
Because of this, France has banned the use of St. John's wort in products. In other countries St. John's wort is only available with a prescription.
Yet, St John's Wort is available in tablets over the counter here in the UK.
The NHS here in the UK points out that,
It should be noted that St. John’s wort is an unlicensed herbal medication, so would not be prescribed by a general practitioner (as the newspapers suggest). St. John’s wort can have serious interactions with a large number of commonly used medications (e.g. warfarin), by affecting enzymes in the body involved with the metabolic processing of drugs; therefore a discussion with a doctor is important. Additionally, St. John’s wort must not be started while another antidepressant is being taken (NHS, 2008).
WebMD also states
St. John's wort is POSSIBLY UNSAFE when taken by mouth in large doses. When taken by mouth in large doses, it might cause severe skin reactions to sun exposure. Women may be at risk of severe skin reactions even at usual doses of St. John's wort. Wear sun block outside, especially if you are light-skinned.
St John's Wort and Major Depression
The NHS covers this too;
The findings [of a systematic review and meta-analysis] – that it is more effective than placebo and the same as standard antidepressants (albeit safer) - apply mainly to people with mild to moderate depression. The authors say that for severe major depression, the evidence is ‘still insufficient to draw conclusions’.
Long Term Use
The answer to your question
Are there any side effects or reduced efficiency of long term use of st Johns wort?
depends on what you call long term for one thing.
Brattström (2009) conducted the study over a 1 year period calling it long term. If you are talking about longer than this, as St John's Wort only helps with mild to moderate depression, I would wonder about the efficacy of St John's Wort in your case. Therefore, I would suggest seeking advice of your doctor again.
As long as you check with your doctor before taking it, plus
- you are not taking other medications which interact badly with St John's Wort, and
- you are not suffering any side effects
then it is generally considered safe (WebMD, 2017) but must be used with caution, in the same dosages prescribed by the doctor or indicated on the packaging.
Brattström, A. (2009). Long-term effects of St. John's wort (Hypericum perforatum) treatment: A 1-year safety study in mild to moderate depression. Phytomedicine, 16(4), 277-283. doi: 10.1016/j.phymed.2008.12.023 pmid: 19299116
British National Formulary (2019) St John's Wort Interactions [Online] Retreived from: https://bnf.nice.org.uk/interaction/st-johns-wort-2.html
NHS (2008) St John's wort for depression [Online] Retrieved from: https://www.nhs.uk/news/mental-health/st-johns-wort-for-depression
WebMD (n.d.) St John's Wort [Online] Retreived from: https://www.webmd.com/vitamins/ai/ingredientmono-329/st-johns-wort
WebMD (2017) St. John's wort - ART-20362212 [Online] Retreived from: https://www.mayoclinic.org/drugs-supplements-st-johns-wort/art-20362212