1. A pharmaceutical composition comprising: i) a safe and therapeutically effective amount of lamivudine or a pharmaceutically acceptable derivative thereof; ii) a safe and therapeutically effective amount of zidovudine or a pharmaceutically acceptable derivative thereof; and iii) a pharmaceutically acceptable glidant, wherein the pharmaceutically acceptable glidant is present in an amount of 0.05% to about 10.0% by weight at which lamivudine, zidovudine and glidant are mixed prior to pill pressing. 2. A pharmaceutical composition according to Claim 1, wherein the pharmaceutically acceptable glidant is selected from a group consisting of: silicon dioxide, colloidal silicon dioxide, fumed silicon dioxide, sodium aluminosilicate, calcium silicate, powdered cellulose, microcrystalline cellulose, corn starch, sodium benzoate, calcium carbonate, magnesium carbonate, asbestos free talc, metallic stearates, calcium stearate, magnesium stearate, zinc stearate, stearowet C, starch, starch 1500, magnesium lauryl sulfate, or magnesium oxide. 3. The pharmaceutical composition according to Claim 2 wherein the pharmaceutically acceptable glidant is fumed silicon dioxide. 4. The pharmaceutical composition according to any one of Claims 1 to 3 wherein the amount of lamivudine is from about 15 to about 1000mg per unit dosage form. 5. The pharmaceutical composition according to Claim 4 wherein the amount of lamivudine is from about 100 to about 500mg per unit dosage form. 6. The pharmaceutical composition according to Claim 5 wherein the amount of lamivudine is 150mg per unit dosage form. 7. The pharmaceutical composition according to any one of Claims 1 to 6 wherein the amount of zidovudine is from about 30 to about 1000mg per unit dosage form. 8. The pharmaceutical composition according to Claim 7 wherein the amount of zidovudine is from about 200 to about 500mg per unit dosage form. 9. The pharmaceutical composition according to Claim 8 wherein the amount of zidovudine is 300mg per unit dosage form. 10. The pharmaceutical composition according to any one of Claims 1 to 9 wherein lamivudine is provided substantially free of the corresponding (+)enantiomer. 11. The pharmaceutical composition according to any one of Claims 1 to 9 wherein the (+)-enantiomer is present in an amount of not more than about 5% w/w of the amount of lamivudine. 12. A pharmaceutical composition according to any one of Claims 1 to 11 wherein the composition is coated with a pharmaceutically acceptable coating. 13. A method for increasing and maintaining the homogeneity of a pharmaceutical composition by including a safe and therapeutically effective amount of lamivudine or a pharmaceutically acceptable derivative thereof and a safe and therapeutically effective amount of zidovudine or a pharmaceutically acceptable derivative thereof at which lamivudine, zidovudine and from 0.05% to about 10.0% by weight of pharmaceutically acceptable of glidant are mixed prior to pill pressing. 14. The method according to claim 13, wherein the glidant is selected from the group consisting of: silicon dioxide, colloidal silicon dioxide, fumed silicon dioxide, sodium aluminosilicate, calcium silicate, powdered cellulose, microcrystalline cellulose, corn starch, sodium benzoate, calcium carbonate, magnesium carbonate, asbestos free talc, metallic stearates, calcium stearate, magnesium stearate, zinc stearate, stearowet C, starch, starch 1500, magnesium lauryl sulfate, or magnesium oxide. 15. The method according to claim 14, wherein the glidant is selected from the group consisting of: fumed silicon dioxide, colloidal silicon dioxide, or fumed colloidal silicon dioxide. 16. A pharmaceutical composition manufactured according to any of claims 1 to 12. 17. The pharmaceutical composition according to claim 16 for treating a retroviral infection, 18. The pharmaceutical composition according to claim 17, wherein the retrovirus is an immunodeficiency virus, including HIV. 19. An article of manufacture comprising: i) packaging material; and ii) a pharmaceutical composition contained within the packaging material according to any of claims 16 to 18. 20. An article of manufacture of Claim 19 additionally comprising a brochure containing product information. 21. An article of manufacture according to Claim 19 or 20, wherein the packaging material is unit dose blister packaging.