Since lemon juice has a pH of 2 and skin's pH is 4.0-7.0, this method needs to be used carefully. If left on too long, or not diluted this can lead to significant chemical burns. Citrus juices also contains a chemical called Bergapten, which binds to DNA and allows UV radiation to damage skin more easily, so you need to watch your sun exposure if you have any citrus juice on your skin. Rinse it off before going into the sun, and wear sunscreen.
Wear sunscreen everyday and do not tan. Ultraviolet Radiation is the number one cause of premature aging. It also leads to skin cancer in high enough doses. Treat the sun like the death ray that it is. Exposing your skin to harmful UVA and UVB rays damages skin and prolongs post inflammatory erythema(PIE)--red acne marks, as the sunlight stimulates pigment-producing cells.
Drugs: Some medications may cause or worsen acne, such as those containing iodides, bromides, or oral or injected steroids (either the medically prescribed prednisone [Deltasone, Orasone, Prednicen-M, Liquid Pred] or the steroids that bodybuilders or athletes sometimes take). Other drugs that can cause or aggravate acne are anticonvulsant medications and lithium (Eskalith, Lithobid). Most cases of acne, however, are not drug related.