Obviously you have had colour before that lasted and your are using the same product of color now, so it sounds like the medication is what is affecting your colour. If it is a dark permanent colour there is no reason for the color to fade off at all. You would or should just get a re-growth and usually a full head color needs a re-touch every 4 weeks as the hair grows approximately 1/2 -inch per month. Sometimes lighter blonde shades can fade off quicker due to the fact the dye load is not as heavy as the darker shades and of course if it is a demi-permanent that also will be a reason for the colour dark or light to fade. Unfortunately usually colour problems or unusual situations are due to something being on the hair, like a medication, or mineral from pipes etc. I would suggest that before you do your colour, to use a chelating shampoo to help take some of the medication away from your hair, apply the color to dry hair. After use a cuticle sealer and do not wash your hair for 24 to 48 hours immediately after the colour, these products mentioned can be found in the JOICO line, not sure this is the answer but it might help?