Alessandra - you really, really don't want to try and reattain a weight you only got to by taking drugs.
The lightest I've been in my adult life was 10 stone, however when I did my 'true weight' info on the Dukan site I didn't input 10 stone because the time I was that weight, it was purely because I was doing amphetamines (to lose weight), working like a nutcase and suffering from glandular fever. That low weight hadn't been gained through proper eating, but by drugs and illness. I'd input whatever the lowest weight was that you achieved *naturally* and then the weight you can maintain easily.
My true weight, as you can see, is 12 stone which many would say is still too heavy. But if I can maintain at that point, it'll be a lot healthier for me in the long run than the yo-yo cycle of fighting to go lower, gaining some back, fighting to get it down again etc etc.