I don't know much about the topic, but here are some research points you can get started with.
For strong magnetic fields, the most notable effect seems to be visual effects (source), called phosphenes (magnetophosphenes in the specific case of magnetic causes) caused by inductance of electric currents in the retina (source).
"Studies" seem to have suggested that 50T fields cause tissue damage, for unspecified reasons (weak source). I could not locate these studies. However, the implication is that immediate death / severe damage is not caused at even 50T fields (for reference, MRIs generally run in the 1.5-3T range).
There are related questions here:
There is an interesting discussion on Reddit:
There is also a field of study called bioelectromagnetics dedicated to biological effects of magnetic fields, which can serve as a good starting point for research:
"Transcranial magnetic stimulation", referenced in both the Reddit and Wikipedia pages, uses small fields in the range 1-10mT to affect the polarization of neurons in the brain.
It seems that the pattern of change of a magnetic field has a more pronounced effect than the strength of the field. Static fields do significantly less (or no) damage, while at high frequencies a weak magnetic field could certainly do significant damage, e.g. a microwave oven.
Primary causes of damage from non-static fields mostly seem to be due to heat, or due to induced electrical current; for example, from the ReviseMRI link above:
A more serious consequence of electric currents flowing through the
body is ventricular fibrillation (though these levels are strictly
prevented in MRI). ... As a general guide, the
faster the imaging or spectroscopy sequence, the greater the rate of
change of the gradient fields used, and the resultant current density
induced in the tissue is higher.
It would doubtless take an extremely strong magnet, higher than anything we could produce, to pull the iron out of your body (conjecture, no source). Note also that there is only about 3-5 grams of iron (something like 2 cm3) in the human body (source, unreferenced source), mostly bound to hemoglobin.
Count Iblis pointed out, in question comments, that there is a nice discussion of magnetars and strong magnetic fields here, which provides nice overviews and plenty of interesting information (although a bit dated):
Fields in excess of 109 Gauss, however, would be instantly lethal.
Such fields strongly distort atoms, compressing atomic electron clouds
into cigar shapes, with the long axis aligned with the field, thus
rendering the chemistry of life impossible. A magnetar within 1000
kilometers would thus kill you via pure static magnetism -- if it
didn't already get you with X-rays, gamma rays, high energy particles,
extreme gravity, bursts and flares...
As for long term effects of more commonly encountered field strengths, there is generally little association between magnetic fields and cancer (source, source).
I hope this helps. Sorry I do not know a direct answer. It certainly depends on more than just the field strength, however.