An article in the Guardian today about how certain algae harvest b12 and that this could be exploited to create a natural source of B12 for vegans. I was a little confused by the following paragraph.
"Supplements based on vitamin B12 are already available but some are effective and others are not, added her colleague, Dr Payam Mehrshahi. “If you are on a vegan diet, you are at risk of being vitamin B12 deficient. You can buy supplements but if you pick up the wrong tablet, the wrong supplement, then you could continue to cause yourself harm.”
I wasn't aware of these ineffective or harmful B12 supements. What are they?
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Budsa
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... Plants and fungi have an alternative methionine synthase (METE) that does not need a B12 cofactor, so these organisms are typically considered to neither synthesise nor utilise B12. In contrast many algal species utilise B12 if it is available, because they encode both METE and METH. Moreover, a large proportion of algal species encode METH only, and so are like animals in being dependent on an external source of the vitamin. ...
Yes but it’s created in the large intestine which is past the terminal ileum where vitamin B12 is absorbed so useless! Prof Tim Spector talks about it in his latest book.
With regard to the specific claim made by Dr. Payam Mehrshahi:
" You can buy supplements but if you pick up the wrong tablet, the wrong supplement, then you could continue to cause yourself harm.”"
If he's talking about B12 supplements specifically, then, unless the supplement does not contain the amount it claims, then there is no risk of harm. Although harmful B12 analogues do exist (they compete with human-usable B12), I have never heard of a case of them being found in a B12 supplement.
You could also get the dose wrong, which is quite easy to do if the absorption mechanisms are not properly understood. As a reminder, any of these is safe and effective:
10mcg 3x day, 100mcg 1 x day, 2000 mcg 1x week.
An insurance policy would be to take two B12 supplements from 2 different suppliers, at different times of the day, that way, even if one has a problem, you are still getting some B12 from the second.
I'm intrigued by the possibility of usable B12 from algae, that might turn out to be much cheaper than the current bacterial synthesis techniques. Duckweed is also an interesting potential B12 source.
The first 2 dosages would only be suitable for vegans and other people with a diet low in natural b12 sources. The recommended oral dose for someone who has an absorption issue is usually 1000 mcg daily, though, generally, the IM route should be used in preference.
I took 1000mcg of oral b12 for 3 years and only raised my serum b12 level from 144 to a maximum of 450 ng/L, though a recent test had it at 860 ng/L so I have now reduced to 500 mch per day.
Sorry if that was unclear. I was recommending dosages for vegans who have no absorption problems. In case of absorption problems, 1000mcg daily would be more appropriate, and for those with PA, many find that oral doses, no matter how high or how frequent, are ineffective at controlling symptoms and resulting in effective healing.
I do not have PA or absorption issues but I needed frequent injections for some months to adequately treat my deficiency.
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Difference between PA and B12 deficiency
B12 deficiency 
High serum B12, High Active B12, Anaemia, Elevated Liver Enzymes (ALP & GGT)
Possible B12 deficiency, looking for recommended test list before B12 shot
