1. A method for treating a mammalian organism to obtain a desired local or systemic physiological or pharmacological effect comprising: administering a sustained release drug delivery system to a mammalian organism in need of such treatment, said drug delivery system comprising; (1) an inner core or reservoir comprising an effective amount of an agent to obtain a desired local or systemic physiological or pharmacological effect, (2) a first coating layer permeable to the passage of said agent, wherein said first coating layer covers at least a portion of said inner core, which provides control and sustained release of the agent, (3) a second coating layer, said second coating layer essentially impermeable to the passage of said agent, and said second coating layer covering at least 50% of the inner core and/or the first coating layer, wherein at least a small portion of the inner core or first coating layer is not coated with said second coating layer and said second coating layer comprises an impermeable film and at least one impermeable disc being different material from or having substantially greater hardness, thickness, malleability or response to heat curing than said impermeable film, and (4) a third coating layer permeable to the passage of said agent, wherein said third coating layer essentially completely covers said second coating layer and the uncoated portion of the first coating layer or inner core, whereby said agent is able to pass through said third coating layer in a controlled manner. 2. A method for treating a mammalian organism as claimed in Claim 1, wherein said third coating layer comprises polyvinyl alcohol. 3. A method for treating a mammalian organism as claimed in Claim 2, wherein said second coating layer comprises ethylene vinyl acetate. 4. A method for treating a mammalian organism as claimed in Claim 3, wherein said first coating layer comprises polyvinyl alcohol. 5. A method for treating a mammalian organism as claimed in Claim 1, wherein said effective agent comprises fluocinolone acetonide, nimodipine or ganciclovir. 6. A method for treating a mammalian organism for cytomegalovirus retinitis comprising: administering a sustained release drug delivery system to a mammalian organism in need of such treatment, said drug delivery system comprising: (1) an inner core or reservoir comprising an effective amount of ganciclovir effective to obtain a desired local or systemic physiological or pharmacological effect, (2) a first coating layer permeable to the passage of said agent, said first coating layer essentially completely covers said inner core, (3) a second coating layer, said second coating layer essentially impermeable to the passage of said ganciclovir, and said second coating layer covering at least 50% of the inner core and/or the first coating layer, wherein at least a small portion of the inner core or first coating layer is not coated with said second coating layer, and wherein said second coating layer comprises an impermeable film and at least one impermeable disc being a different material from or having substantially greater hardness, thickness, malleability or response to heat curing than said impermeable film, and (4) a third coating layer permeable to the passage of said ganciclovir, wherein said third coating layer essentially completely covers said second coating layer and the uncoated portion of the first coating layer or inner core, whereby said ganciclovir is able to pass through said third coating layer in a controlled manner. 7. A method for providing controlled and sustained administration of an agent effective in obtaining a desired local or systemic physiological or pharmacological effect comprising: surgically implanting a sustained release drug delivery system at a desired location, said drug delivery system comprising; (1) an inner core or reservoir comprising an effective amount of an agent effective in obtaining a desired physiological or pharmacological effect, and (2) a first coating layer permeable to the passage of said agent, wherein said first coating layer covers at least a portion of said inner core, which provides control and sustained release of the agent, (3) a second coating layer, said second coating layer essentially impermeable to the passage of said agent, and said second coating layer covering at least 50% of the inner core and/or the first coating layer, wherein at least a small portion of the inner core or first coating layer is not coated with said second coating layer and said second coating layer comprises an impermeable film and at least one impermeable disc being a different material from or having substantially greater hardness, thickness, malleability or response to heat curing than said impermeable film, and (4) a third coating layer permeable to the passage of said agent, wherein said third coating layer essentially completely covers said second coating layer and the uncoated portion of the first coating layer or inner core, whereby said agent is able to pass through said third coating layer in a controlled manner. 8. A method as claimed in Claim 7, wherein said device is surgically implanted within the vitreous of the eye. 9. A method as claimed in Claim 7, wherein said first and third coating layers comprise polyvinyl alcohol. 10. A method as claimed in Claim 7 wherein said third coating layer is silicone. 11. A method as claimed in Claim 9, wherein said second coating layer comprises ethylene vinyl acetate. 12. A method as claimed in Claim 11, wherein said effective agent is ganciclovir or 5-fluorouracil. 13. A method as claimed in Claim 11 wherein said effective agent is a low solubility steroid. 14. A method as claimed in Claim 13, wherein said low solubility steroid is fluocinolone acetonide. 15. A method as claimed in Claim 11 wherein said effective agent is a neuroprotectant. 16. A method as claimed in Claim 15, wherein said neuroprotectant is nimodipine. 17. A sustained release drug delivery system comprising: (A) an inner core or reservoir comprising an effective amount of an agent effective in obtaining a desired local or systemic physiological or pharmacological effect, (B) a first coating layer permeable to the passage of said agent, wherein said first coating layer covers at least a portion of said inner core, which provides control and sustained release of the agent, (C) a second coating layer, said second coating layer impermeable to the passage of said effective agent, and said second coating layer covering at least 50% of the inner core and/or the first coating layer wherein at least a small portion of the inner core or first coating layer is not coated with said second coating layer and said second coating layer comprises an impermeable film and at least one impermeable disc being a different material from or having substantially greater hardness, thickness, malleability or response to heat curing than said impermeable film, and (D) a third coating layer, said third coating layer permeable to the passage of said effective agent, wherein said third coating layer essentially completely covers the second coating layer and the uncoated portion of the first coating layer or inner core. 18. A sustained release drug delivery system as claimed in Claim 17, wherein said third coating layer comprises polyvinyl alcohol. 19. A sustained release drug delivery system as claimed in Claim 18, wherein said second coating layer comprises ethylene vinyl acetate. 20. A sustained release drug delivery system as claimed in Claim 19, wherein said first coating layer comprises polyvinyl alcohol. 21. A sustained release drug delivery system as claimed in Claim 20, wherein said effective agent is ganciclovir or fluocinolone acetonide. 22. A sustained release drug delivery system as claimed in Claim 17, wherein said effective agent is nimodipine. 23. A sustained release drug delivery system as claimed in Claim 17, wherein the third coating layer comprises silicone. 24. A method for treating a mammalian organism according to Claim 1, wherein said impermeable film is made of a composition different from said impermeable disc. 25. A method for treating a mammalian organism as claimed in Claim 1, wherein said second coating layer covers all of the inner core and first coating layer except for a diffusion port opening. 26. A method for treating a mammalian organism as claimed in Claim 1, wherein said impermeable disc is harder than said impermeable film. 27. The sustained release drug delivery system as claimed in Claim 17, wherein said effective agent is a steroid. 28. The sustained drug delivery system as claimed in Claim 17, wherein said effective agent is a neuroprotectant.