To answer this question, we have to go back to the basics of the cycle of hair growth, resting and shedding. When you are born, there are more than one million hair follicles on your head with one-hundred-thousand on the scalp. That's a lot of hair! The number does not change over your lifetime but, as the head grows, the follicles are spread out more. Each hair grows about 6 inches a year but, randomly, the hairs are in different stages. The active period of growth is called anagen where the cells divide and form new growth over a period of between two and six years. The hair then goes into the catagen stage. At any one time, about 3% of all your scalp hair is in a transitional state for about three weeks. The final stage is telogen. About 7% of all your scalp hair rests for about three months. Towards the end of this stage, between 25 and 100 hairs are shed each day.
From this you will understand the problem of an "immediate" halt in any part of this process. We are talking about one-hundred-thousand hairs and 90% of those hairs are growing. To ensure the coverage of the head remains even, the distribution of the 10% that will slowly transition and fall out is entirely random. Thus, when everything is working properly, only a tiny percentage of hair is falling from each part of the scalp on any given day. If you are diagnosed with male pattern baldness, the distribution process has broken down and more hair is lost from particular parts of the scalp, thereby forming the distinctive pattern. The effect of the drug is to move the body's chemistry back in time and trigger the more random process of selecting hairs for the catagen and telogen stages. This is not something that can happen immediately. It takes time to change the body's chemistry. Indeed, if you were to try accelerating the process and took too much of the active ingredients, there would be no change to the process of hair growth and shedding, but there could be quite serious effects on other parts of the body (in animal tests, rats and mice die).
In the clinical trials and all experience following licensing by the FDA, there's no visible reaction for at least three months after starting treatment with Propecia. This is not to say the drug is inactive for the initial period of time. But to warn you there can be significant delays before you see any results. It's not uncommon for there to be no obvious reaction for six months. Some men wait for a year before seeing a change. This is a treatment regime that requires patience. Do not believe any drug or product manufacturer who attempts to persuade you that parting with your money will produce instant results. This is a scam. Propecia works slowly and steadily to reduce the amount of dihydrotestosterone. It's not a scam.