Taking your 99% effectiveness of the pill, the probability of the failure of that is 1%, or 0.01. The probability of failure of the pull-out method (done correctly) is 4%, or 0.04. Combining the two, means the probability of simultaneous failure is 0.01*0.04 = 0.0004, or 0.04%, or 1 out of 2500 times.
This is in the form of retinal activity equivalents to account for the various sources of vitamin A. The best sources of preformed vitamin A are in liver and fish oils. This includes beef, calf, chicken liver and cod liver oil.
Although itâs possible to get too little vitamin A, itâs easy to get too much preformed vitamin A (retinol) from supplements. Intake of up to 3,000 micrograms of preformed vitamin A, more than three times the current recommended daily level, is thought to be safe. However, there is some evidence that this much preformed vitamin A might increase the risk of hip fracture (1-3) or some birth defects.